CVE-2024-42295 nilfs2: handle inconsistent state in nilfs_btnode_create_block()

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

nilfs2: handle inconsistent state in nilfs_btnode_create_block()

Syzbot reported that a buffer state inconsistency was detected in
nilfs_btnode_create_block(), triggering a kernel bug.

It is not appropriate to treat this inconsistency as a bug; it can occur
if the argument block address (the buffer index of the newly created
block) is a virtual block number and has been reallocated due to
corruption of the bitmap used to manage its allocation state.

So, modify nilfs_btnode_create_block() and its callers to treat it as a
possible filesystem error, rather than triggering a kernel bug.