fscrypt: explicitly require that inode->i_blkbits be set

Document that fscrypt_prepare_new_inode() requires inode->i_blkbits to
be set, and make it WARN if it's not.  This would have made the CephFS
bug https://tracker.ceph.com/issues/64035

 a bit easier to debug.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Link: https://lore.kernel.org/r/20240201003525.1788594-1-xiubli@redhat.com


Signed-off-by: Eric Biggers <ebiggers@google.com>