btrfs: only enforce free space tree if v1 cache is required for bs < ps cases

[BUG]
Since the introduction of btrfs bs < ps support, v1 cache was never on
the plan due to its hard coded PAGE_SIZE usage, and the future plan to
properly deprecate it.

However for bs < ps cases, even if 'nospace_cache,clear_cache' mount
option is specified, it's never respected and free space tree is always
enabled:

mkfs.btrfs -f -O ^bgt,fst $dev
mount $dev $mnt -o clear_cache,nospace_cache
umount $mnt
btrfs ins dump-super $dev
...
compat_ro_flags 0x3
( FREE_SPACE_TREE |
FREE_SPACE_TREE_VALID )
...

This means a different behavior compared to bs >= ps cases.

[CAUSE]
The forcing usage of v2 space cache is done inside
btrfs_set_free_space_cache_settings(), however it never checks if we're
even using space cache but always enabling v2 cache.

[FIX]
Instead unconditionally enable v2 cache, only forcing v2 cache if the
old v1 cache is required.

Now v2 space cache can be properly disabled on bs < ps cases:

mkfs.btrfs -f -O ^bgt,fst $dev
mount $dev $mnt -o clear_cache,nospace_cache
umount $mnt
btrfs ins dump-super $dev
...
compat_ro_flags 0x0
...

Fixes: 9f73f1aef98b ("btrfs: force v2 space cache usage for subpage mount")
Reviewed-by: Filipe Manana <[e-mail]>
Signed-off-by: Qu Wenruo <[e-mail]>
Reviewed-by: David Sterba <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/super.c

···
  */
 void btrfs_set_free_space_cache_settings(struct btrfs_fs_info *fs_info)
 {
-	if (fs_info->sectorsize < PAGE_SIZE) {
+	if (fs_info->sectorsize < PAGE_SIZE && btrfs_test_opt(fs_info, SPACE_CACHE)) {
+		btrfs_info(fs_info,
+			   "forcing free space tree for sector size %u with page size %lu",
+			   fs_info->sectorsize, PAGE_SIZE);
 		btrfs_clear_opt(fs_info->mount_opt, SPACE_CACHE);
-		if (!btrfs_test_opt(fs_info, FREE_SPACE_TREE)) {
-			btrfs_info(fs_info,
-				   "forcing free space tree for sector size %u with page size %lu",
-				   fs_info->sectorsize, PAGE_SIZE);
-			btrfs_set_opt(fs_info->mount_opt, FREE_SPACE_TREE);
-		}
+		btrfs_set_opt(fs_info->mount_opt, FREE_SPACE_TREE);
 	}
 
 	/*