xfs: refactor converting btree irec to btree key

We keep doing these conversions to support btree queries, so refactor
this into a helper.

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