btrfs: factor out helper for single device IO check

The check in btrfs_map_block() deciding if a particular I/O is targeting a
single device is getting more and more convoluted.

Factor out the check conditions into a helper function, with no functional
change otherwise.
Reviewed-by: Christoph Hellwig <hch@lst.de>
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

@@ -6330,6 +6330,27 @@ static int set_io_stripe(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
 	return 0;
 }
 
+static bool is_single_device_io(struct btrfs_fs_info *fs_info,
+				const struct btrfs_io_stripe *smap,
+				const struct btrfs_chunk_map *map,
+				int num_alloc_stripes,
+				enum btrfs_map_op op, int mirror_num)
+{
+	if (!smap)
+		return false;
+
+	if (num_alloc_stripes != 1)
+		return false;
+
+	if (btrfs_need_stripe_tree_update(fs_info, map->type) && op != BTRFS_MAP_READ)
+		return false;
+
+	if ((map->type & BTRFS_BLOCK_GROUP_RAID56_MASK) && mirror_num > 1)
+		return false;
+
+	return true;
+}
+
 /*
  * Map one logical range to one or more physical ranges.
  *
@@ -6532,10 +6553,8 @@ int btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
 	 * physical block information on the stack instead of allocating an
 	 * I/O context structure.
 	 */
-	if (smap && num_alloc_stripes == 1 &&
-	    !(btrfs_need_stripe_tree_update(fs_info, map->type) &&
-	      op != BTRFS_MAP_READ) &&
-	    !((map->type & BTRFS_BLOCK_GROUP_RAID56_MASK) && mirror_num > 1)) {
+	if (is_single_device_io(fs_info, smap, map, num_alloc_stripes, op,
+				mirror_num)) {
 		ret = set_io_stripe(fs_info, op, logical, length, smap, map,
 				    stripe_index, stripe_offset, stripe_nr);
 		if (mirror_num_ret)