btrfs: remove redundant value assignment at btrfs_add_extent_mapping()

At btrfs_add_extent_mapping(), in case add_extent_mapping() returned
-EEXIST, it's pointless to assign 0 to 'ret' since we will assign a value
to it shortly after, without 'ret' being used before that. So remove that
pointless assignment.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_map.c

@@ -586,8 +586,6 @@ int btrfs_add_extent_mapping(struct btrfs_fs_info *fs_info,
 	if (ret == -EEXIST) {
 		struct extent_map *existing;
 
-		ret = 0;
-
 		existing = search_extent_mapping(em_tree, start, len);
 
 		trace_btrfs_handle_em_exist(fs_info, existing, em, start, len);