xfs: don't track firstrec/firstkey separately in xchk_btree

The btree scrubbing code checks that the records (or keys) that it finds
in a btree block are all in order by calling the btree cursor's
->recs_inorder function.  This of course makes no sense for the first
item in the block, so we switch that off with a separate variable in
struct xchk_btree.

Christoph helped me figure out that the variable is unnecessary, since
we just accessed bc_ptrs[level] and can compare that against zero.  Use
that, and save ourselves some memory space.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>