nilfs2: prevent use of deleted inode

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

nilfs2: prevent use of deleted inode

syzbot reported a WARNING in nilfs_rmdir. [1]

Because the inode bitmap is corrupted, an inode with an inode number that

should exist as a ".nilfs" file was reassigned by nilfs_mkdir for "file0",

causing an inode duplication during execution. And this causes an

underflow of i_nlink in rmdir operations.

The inode is used twice by the same task to unmount and remove directories

".nilfs" and "file0", it trigger warning in nilfs_rmdir.

Avoid to this issue, check i_nlink in nilfs_iget(), if it is 0, it means

that this inode has been deleted, and iput is executed to reclaim it.

[1]

WARNING: CPU: 1 PID: 5824 at fs/inode.c:407 drop_nlink+0xc4/0x110 fs/inode.c:407

...

Call Trace:

<TASK>

nilfs_rmdir+0x1b0/0x250 fs/nilfs2/namei.c:342

vfs_rmdir+0x3a3/0x510 fs/namei.c:4394

do_rmdir+0x3b5/0x580 fs/namei.c:4453

__do_sys_rmdir fs/namei.c:4472 [inline]

__se_sys_rmdir fs/namei.c:4470 [inline]

__x64_sys_rmdir+0x47/0x50 fs/namei.c:4470

do_syscall_x64 arch/x86/entry/common.c:52 [inline]

do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83

entry_SYSCALL_64_after_hwframe+0x77/0x7f