btrfs: move mapping of block for discard to its caller

There's a simple forwarded call based on the operation that would better
fit the caller btrfs_map_block that's until now a trivial wrapper.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -5878,10 +5878,7 @@ static int __btrfs_map_block(struct btrfs_fs_info *fs_info,
 	struct btrfs_io_geometry geom;
 
 	ASSERT(bbio_ret);
-
-	if (op == BTRFS_MAP_DISCARD)
-		return __btrfs_map_block_for_discard(fs_info, logical,
-						     length, bbio_ret);
+	ASSERT(op != BTRFS_MAP_DISCARD);
 
 	ret = btrfs_get_io_geometry(fs_info, op, logical, *length, &geom);
 	if (ret < 0)
@@ -6111,6 +6108,10 @@ int btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
 		      u64 logical, u64 *length,
 		      struct btrfs_bio **bbio_ret, int mirror_num)
 {
+	if (op == BTRFS_MAP_DISCARD)
+		return __btrfs_map_block_for_discard(fs_info, logical,
+						     length, bbio_ret);
+
 	return __btrfs_map_block(fs_info, op, logical, length, bbio_ret,
 				 mirror_num, 0);
 }