nfs: Clean up /proc/net/rpc/nfs when nfs_fs_proc_net_init() fails.

In the Linux kernel, the following vulnerability has been resolved:

nfs: Clean up /proc/net/rpc/nfs when nfs_fs_proc_net_init() fails.

syzbot reported a warning below [1] following a fault injection in
nfs_fs_proc_net_init(). [0]

When nfs_fs_proc_net_init() fails, /proc/net/rpc/nfs is not removed.

Later, rpc_proc_exit() tries to remove /proc/net/rpc, and the warning
is logged as the directory is not empty.

Let's handle the error of nfs_fs_proc_net_init() properly.

[0]:
FAULT_INJECTION: forcing a failure.
name failslab, interval 1, probability 0, space 0, times 0
CPU: 1 UID: 0 PID: 6120 Comm: syz.2.27 Not tainted 6.16.0-rc1-syzkaller-00010-g2c4a1f3fe03e #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
Call Trace:
<TASK>
dump_stack_lvl (lib/dump_stack.c:123)
should_fail_ex (lib/fault-inject.c:73 lib/fault-inject.c:174)
should_failslab (mm/failslab.c:46)
kmem_cache_alloc_noprof (mm/slub.c:4178 mm/slub.c:4204)
__proc_create (fs/proc/generic.c:427)
proc_create_reg (fs/proc/generic.c:554)
proc_create_net_data (fs/proc/proc_net.c:120)
nfs_fs_proc_net_init (fs/nfs/client.c:1409)
nfs_net_init (fs/nfs/inode.c:2600)
ops_init (net/core/net_namespace.c:138)
setup_net (net/core/net_namespace.c:443)
copy_net_ns (net/core/net_namespace.c:576)
create_new_namespaces (kernel/nsproxy.c:110)
unshare_nsproxy_namespaces (kernel/nsproxy.c:218 (discriminator 4))
ksys_unshare (kernel/fork.c:3123)
__x64_sys_unshare (kernel/fork.c:3190)
do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/x86/entry/syscall_64.c:94)
entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
</TASK>

[1]:
remove_proc_entry: removing non-empty directory 'net/rpc', leaking at least 'nfs'
WARNING: CPU: 1 PID: 6120 at fs/proc/generic.c:727 remove_proc_entry+0x45e/0x530 fs/proc/generic.c:727
Modules linked in:
CPU: 1 UID: 0 PID: 6120 Comm: syz.2.27 Not tainted 6.16.0-rc1-syzkaller-00010-g2c4a1f3fe03e #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
RIP: 0010:remove_proc_entry+0x45e/0x530 fs/proc/generic.c:727
Code: 3c 02 00 0f 85 85 00 00 00 48 8b 93 d8 00 00 00 4d 89 f0 4c 89 e9 48 c7 c6 40 ba a2 8b 48 c7 c7 60 b9 a2 8b e8 33 81 1d ff 90 <0f> 0b 90 90 e9 5f fe ff ff e8 04 69 5e ff 90 48 b8 00 00 00 00 00
RSP: 0018:ffffc90003637b08 EFLAGS: 00010282
RAX: 0000000000000000 RBX: ffff88805f534140 RCX: ffffffff817a92c8
RDX: ffff88807da99e00 RSI: ffffffff817a92d5 RDI: 0000000000000001
RBP: ffff888033431ac0 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000001 R12: ffff888033431a00
R13: ffff888033431ae4 R14: ffff888033184724 R15: dffffc0000000000
FS: 0000555580328500(0000) GS:ffff888124a62000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f71733743e0 CR3: 000000007f618000 CR4: 00000000003526f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
sunrpc_exit_net+0x46/0x90 net/sunrpc/sunrpc_syms.c:76
ops_exit_list net/core/net_namespace.c:200 [inline]
ops_undo_list+0x2eb/0xab0 net/core/net_namespace.c:253
setup_net+0x2e1/0x510 net/core/net_namespace.c:457
copy_net_ns+0x2a6/0x5f0 net/core/net_namespace.c:574
create_new_namespaces+0x3ea/0xa90 kernel/nsproxy.c:110
unshare_nsproxy_namespaces+0xc0/0x1f0 kernel/nsproxy.c:218
ksys_unshare+0x45b/0xa40 kernel/fork.c:3121
__do_sys_unshare kernel/fork.c:3192 [inline]
__se_sys_unshare kernel/fork.c:3190 [inline]
__x64_sys_unshare+0x31/0x40 kernel/fork.c:3190
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xcd/0x490 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fa1a6b8e929
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c
---truncated---