xfs: consolidate btree block verification

Add a __xfs_btree_check_block helper that can be called by the scrub code
to validate a btree block of any form, and move the duplicate error
handling code from xfs_btree_check_sblock and xfs_btree_check_lblock into
xfs_btree_check_block and thus remove these two helpers.

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>