xfs: actually check xfs_btree_check_block return in xfs_btree_islastblock

Coverity points out that xfs_btree_islastblock doesn't check the return
value of xfs_btree_check_block.  Since the question "Does the cursor
point to the last block in this level?" only makes sense if the caller
previously performed a lookup or seek operation, the block should
already have been checked.

Therefore, check the return value in an ASSERT and turn the whole thing
into a static inline predicate.

Coverity-id: 114069
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>