btrfs: qgroup: avoid memory allocation if qgroups are not enabled

At btrfs_qgroup_inherit() we allocate a qgroup record even if qgroups are
not enabled, which is unnecessary overhead and can result in subvolume
and snapshot creation to fail with -ENOMEM, as create_subvol() calls this
function.

Improve on this by making btrfs_qgroup_inherit() check if qgroups are
enabled earlier and return if they are not, avoiding the unnecessary
memory allocation and taking some locks.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: fdmanana

fs/btrfs/qgroup.c

@@ -3328,6 +3328,9 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans, u64 srcid,
 	u32 level_size = 0;
 	u64 nums;
 
+	if (!btrfs_qgroup_enabled(fs_info))
+		return 0;
+
 	prealloc = kzalloc(sizeof(*prealloc), GFP_NOFS);
 	if (!prealloc)
 		return -ENOMEM;
@@ -3351,8 +3354,6 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans, u64 srcid,
 
 	if (!committing)
 		mutex_lock(&fs_info->qgroup_ioctl_lock);
-	if (!btrfs_qgroup_enabled(fs_info))
-		goto out;
 
 	quota_root = fs_info->quota_root;
 	if (!quota_root) {