nilfs2: fix infinite loop in nilfs_mdt_get_block()

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

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.