ceph: always set initial i_blkbits to CEPH_FSCRYPT_BLOCK_SHIFT

The fscrypt code will use i_blkbits to setup ci_data_unit_bits when
allocating the new inode, but ceph will initiate i_blkbits ater when
filling the inode, which is too late. Since ci_data_unit_bits will only
be used by the fscrypt framework so initiating i_blkbits with
CEPH_FSCRYPT_BLOCK_SHIFT is safe.

Link: https://tracker.ceph.com/issues/64035


Fixes: 5b118884 ("fscrypt: support crypto data unit size less than filesystem block size")
Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>