xfs: misc cleanups for __xfs_btree_check_sblock

Remove the local crc variable that is only used once and remove the bp
NULL checking as it can't ever be NULL for short form blocks.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>