xfs: fix warnings about unused stack variables

Reduce stack usage and get rid of compiler warnings by eliminating
unused variables.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>

fs/xfs/libxfs/xfs_bmap.c

@@ -1280,7 +1280,6 @@ xfs_bmap_read_extents(
 		xfs_bmbt_rec_t	*frp;
 		xfs_fsblock_t	nextbno;
 		xfs_extnum_t	num_recs;
-		xfs_extnum_t	start;
 
 		num_recs = xfs_btree_get_numrecs(block);
 		if (unlikely(i + num_recs > room)) {
@@ -1303,7 +1302,6 @@ xfs_bmap_read_extents(
 		 * Copy records into the extent records.
 		 */
 		frp = XFS_BMBT_REC_ADDR(mp, block, 1);
-		start = i;
 		for (j = 0; j < num_recs; j++, i++, frp++) {
 			xfs_bmbt_rec_host_t *trp = xfs_iext_get_ext(ifp, i);
 			trp->l0 = be64_to_cpu(frp->l0);