hfs: fix missing hfs_bnode_get() in __hfs_bnode_create

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

hfs: fix missing hfs_bnode_get() in __hfs_bnode_create

Syzbot found a kernel BUG in hfs_bnode_put():

kernel BUG at fs/hfs/bnode.c:466!
invalid opcode: 0000 [#1] PREEMPT SMP KASAN
CPU: 0 PID: 3634 Comm: kworker/u4:5 Not tainted 6.1.0-rc7-syzkaller-00190-g97ee9d1c1696 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/26/2022
Workqueue: writeback wb_workfn (flush-7:0)
RIP: 0010:hfs_bnode_put+0x46f/0x480 fs/hfs/bnode.c:466
Code: 8a 80 ff e9 73 fe ff ff 89 d9 80 e1 07 80 c1 03 38 c1 0f 8c a0 fe ff ff 48 89 df e8 db 8a 80 ff e9 93 fe ff ff e8 a1 68 2c ff <0f> 0b e8 9a 68 2c ff 0f 0b 0f 1f 84 00 00 00 00 00 55 41 57 41 56
RSP: 0018:ffffc90003b4f258 EFLAGS: 00010293
RAX: ffffffff825e318f RBX: 0000000000000000 RCX: ffff8880739dd7c0
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffffc90003b4f430 R08: ffffffff825e2d9b R09: ffffed10045157d1
R10: ffffed10045157d1 R11: 1ffff110045157d0 R12: ffff8880228abe80
R13: ffff88807016c000 R14: dffffc0000000000 R15: ffff8880228abe00
FS: 0000000000000000(0000) GS:ffff8880b9800000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fa6ebe88718 CR3: 000000001e93d000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
hfs_write_inode+0x1bc/0xb40
write_inode fs/fs-writeback.c:1440 [inline]
__writeback_single_inode+0x4d6/0x670 fs/fs-writeback.c:1652
writeback_sb_inodes+0xb3b/0x18f0 fs/fs-writeback.c:1878
__writeback_inodes_wb+0x125/0x420 fs/fs-writeback.c:1949
wb_writeback+0x440/0x7b0 fs/fs-writeback.c:2054
wb_check_start_all fs/fs-writeback.c:2176 [inline]
wb_do_writeback fs/fs-writeback.c:2202 [inline]
wb_workfn+0x827/0xef0 fs/fs-writeback.c:2235
process_one_work+0x877/0xdb0 kernel/workqueue.c:2289
worker_thread+0xb14/0x1330 kernel/workqueue.c:2436
kthread+0x266/0x300 kernel/kthread.c:376
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:306
</TASK>

The BUG_ON() is triggered at here:

/* Dispose of resources used by a node */
void hfs_bnode_put(struct hfs_bnode *node)
{
if (node) {
<skipped>
BUG_ON(!atomic_read(&node->refcnt)); <- we have issue here!!!!
<skipped>
}
}

By tracing the refcnt, I found the node is created by hfs_bmap_alloc()
with refcnt 1. Then the node is used by hfs_btree_write(). There is a
missing of hfs_bnode_get() after find the node. The issue happened in
following path:

<alloc>
hfs_bmap_alloc
hfs_bnode_find
__hfs_bnode_create <- allocate a new node with refcnt 1.
hfs_bnode_put <- decrease the refcnt

<write>
hfs_btree_write
hfs_bnode_find
__hfs_bnode_create
hfs_bnode_findhash <- find the node without refcnt increased.
hfs_bnode_put <- trigger the BUG_ON() since refcnt is 0.