btrfs: drop unused parameter from extent_map_tree_init

the GFP flags are not stored anywhere and all allocations are done via
alloc_extent_map(GFP_NOFS).

Signed-off-by: David Sterba <dsterba@suse.cz>

fs/btrfs/volumes.c

@@ -2849,7 +2849,7 @@ int btrfs_chunk_readonly(struct btrfs_root *root, u64 chunk_offset)
 
 void btrfs_mapping_init(struct btrfs_mapping_tree *tree)
 {
-	extent_map_tree_init(&tree->map_tree, GFP_NOFS);
+	extent_map_tree_init(&tree->map_tree);
 }
 
 void btrfs_mapping_tree_free(struct btrfs_mapping_tree *tree)