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.