quota: Treat superblock owner as privilged

A user privileged towards s_user_ns is privileged towards the
superblock and should be allowed to perform privileged quota
operations on that superblock. Convert check_quotactl_permission
to check for CAP_SYS_ADMIN in s_user_ns for these operations.
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>

fs/quota/quota.c

@@ -38,7 +38,7 @@ static int check_quotactl_permission(struct super_block *sb, int type, int cmd,
 			break;
 		/*FALLTHROUGH*/
 	default:
-		if (!capable(CAP_SYS_ADMIN))
+		if (!ns_capable(sb->s_user_ns, CAP_SYS_ADMIN))
 			return -EPERM;
 	}