btrfs-progs: mkfs: mark block-group-tree as safe since 6.6

As reported, the 'safe' version of BGT is missing in mkfs output. The
feature has been around for some time, there are no major problems
known. Let's pick the next stable branch after the first release.

Issue: #1044
Signed-off-by: David Sterba <[email protected]>

Author: kdave

common/fsfeatures.c

@@ -218,7 +218,7 @@ static const struct btrfs_feature mkfs_features[] = {
 		.compat_ro_flag	= BTRFS_FEATURE_COMPAT_RO_BLOCK_GROUP_TREE,
 		.sysfs_name	= "block_group_tree",
 		VERSION_TO_STRING2(compat, 6,1),
-		VERSION_NULL(safe),
+		VERSION_NULL(safe, 6,6),
 		VERSION_NULL(default),
 		.desc		= "block group tree, more efficient block group tracking to reduce mount time"
 	},