btrfs: reduce scope of data_stripes in btrfs_map_block

Reduce the scope of 'data_stripes' in btrfs_map_block(). While the
change alone may not make too much sense, it helps us factoring out a
helper function for the block mapping of RAID56 I/O.
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -6478,7 +6478,6 @@ int btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
 	struct btrfs_chunk_map *map;
 	struct btrfs_io_geometry io_geom = { 0 };
 	u64 map_offset;
-	int data_stripes;
 	int i;
 	int ret = 0;
 	int num_copies;
@@ -6503,8 +6502,6 @@ int btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
 	if (IS_ERR(map))
 		return PTR_ERR(map);
 
-	data_stripes = nr_data_stripes(map);
-
 	map_offset = logical - map->start;
 	io_geom.raid56_full_stripe_start = (u64)-1;
 	max_len = btrfs_max_io_len(map, io_geom.op, map_offset, &io_geom.stripe_nr,
@@ -6530,6 +6527,8 @@ int btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
 	} else if (map->type & BTRFS_BLOCK_GROUP_RAID10) {
 		map_blocks_raid10(fs_info, map, &io_geom, dev_replace_is_ongoing);
 	} else if (map->type & BTRFS_BLOCK_GROUP_RAID56_MASK) {
+		int data_stripes = nr_data_stripes(map);
+
 		if (op != BTRFS_MAP_READ || io_geom.mirror_num > 1) {
 			/*
 			 * Needs full stripe mapping.
@@ -6641,7 +6640,8 @@ int btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
 		 * modulo, to reduce one modulo call.
 		 */
 		bioc->full_stripe_logical = map->start +
-			btrfs_stripe_nr_to_offset(io_geom.stripe_nr * data_stripes);
+			btrfs_stripe_nr_to_offset(io_geom.stripe_nr *
+						  nr_data_stripes(map));
 		for (int i = 0; i < io_geom.num_stripes; i++) {
 			ret = set_io_stripe(fs_info, op, logical, length,
 					    &bioc->stripes[i], map,