nilfs2: fix infinite loop in nilfs_mdt_get_block()

If the disk image that nilfs2 mounts is corrupted and a virtual block
address obtained by block lookup for a metadata file is invalid,
nilfs_bmap_lookup_at_level() may return the same internal return code as
-ENOENT, meaning the block does not exist in the metadata file.

This duplication of return codes confuses nilfs_mdt_get_block(), causing
it to read and create a metadata block indefinitely.

In particular, if this happens to the inode metadata file, ifile,
semaphore i_rwsem can be left held, causing task hangs in lock_mount.

Fix this issue by making nilfs_bmap_lookup_at_level() treat virtual block
address translation failures with -ENOENT as metadata corruption instead
of returning the error code.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Ryusuke Konishi <[email protected]>
Tested-by: Ryusuke Konishi <[email protected]>
Reported-by: [email protected]
  Link: https://syzkaller.appspot.com/bug?extid=221d75710bde87fa0e97
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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