xfs: simplify xfs_btree_check_sblock_siblings

Stop using xfs_btree_check_sptr in xfs_btree_check_sblock_siblings,
as it only duplicates the xfs_verify_agbno call in the other leg of
if / else besides adding a tautological level check.

With this the cur and level arguments can be removed as they are
now unused.

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>