nfsd: Fix cred ref leak in nfsd_nl_threads_set_doit().

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

nfsd: Fix cred ref leak in nfsd_nl_threads_set_doit().

syzbot reported memory leak of struct cred. [0]

nfsd_nl_threads_set_doit() passes get_current_cred() to
nfsd_svc(), but put_cred() is not called after that.

The cred is finally passed down to _svc_xprt_create(),
which calls get_cred() with the cred for struct svc_xprt.

The ownership of the refcount by get_current_cred() is not
transferred to anywhere and is just leaked.

nfsd_svc() is also called from write_threads(), but it does
not bump file->f_cred there.

nfsd_nl_threads_set_doit() is called from sendmsg() and
current->cred does not go away.

Let's use current_cred() in nfsd_nl_threads_set_doit().

[0]:
BUG: memory leak
unreferenced object 0xffff888108b89480 (size 184):
comm "syz-executor", pid 5994, jiffies 4294943386
hex dump (first 32 bytes):
01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
backtrace (crc 369454a7):
kmemleak_alloc_recursive include/linux/kmemleak.h:44 [inline]
slab_post_alloc_hook mm/slub.c:4958 [inline]
slab_alloc_node mm/slub.c:5263 [inline]
kmem_cache_alloc_noprof+0x412/0x580 mm/slub.c:5270
prepare_creds+0x22/0x600 kernel/cred.c:185
copy_creds+0x44/0x290 kernel/cred.c:286
copy_process+0x7a7/0x2870 kernel/fork.c:2086
kernel_clone+0xac/0x6e0 kernel/fork.c:2651
__do_sys_clone+0x7f/0xb0 kernel/fork.c:2792
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xa4/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f