btrfs: remove unnecessary mutex lock in qgroup_account_snapshot

The quota status used to be tracked as a variable, so the mutex was
needed (until "Btrfs: add a flags field to btrfs_fs_info" afcdd129).
Since the status is a bit modified atomically and we don't hold the
mutex beyond the check, we can drop it.
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/transaction.c

@@ -1364,12 +1364,8 @@ static int qgroup_account_snapshot(struct btrfs_trans_handle *trans,
 	 * enabled. If this check races with the ioctl, rescan will
 	 * kick in anyway.
 	 */
-	mutex_lock(&fs_info->qgroup_ioctl_lock);
-	if (!test_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags)) {
-		mutex_unlock(&fs_info->qgroup_ioctl_lock);
+	if (!test_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags))
 		return 0;
-	}
-	mutex_unlock(&fs_info->qgroup_ioctl_lock);
 
 	/*
 	 * We are going to commit transaction, see btrfs_commit_transaction()