nilfs2: reject zero bd_oblocknr in nilfs_ioctl_mark_blocks_dirty()

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

nilfs2: reject zero bd_oblocknr in nilfs_ioctl_mark_blocks_dirty()

nilfs_ioctl_mark_blocks_dirty() uses bd_oblocknr to detect dead blocks

by comparing it with the current block number bd_blocknr. If they differ,

the block is considered dead and skipped.

However, bd_oblocknr should never be 0 since block 0 typically stores the

primary superblock and is never a valid GC target block. A corrupted ioctl

request with bd_oblocknr set to 0 causes the comparison to incorrectly

match when the lookup returns -ENOENT and sets bd_blocknr to 0, bypassing

the dead block check and calling nilfs_bmap_mark() on a non-existent

block. This causes nilfs_btree_do_lookup() to return -ENOENT, triggering

the WARN_ON(ret == -ENOENT).

Fix this by rejecting ioctl requests with bd_oblocknr set to 0 at the

beginning of each iteration.

[ryusuke: slightly modified the commit message and comments for accuracy]