Revert "(namespace) quota: Handle quota data stored in s_user_ns in quota_setxquota"

BugLink: https://bugs.launchpad.net/bugs/1644165

This reverts commit 0beac1b4.

The kernel fix for bug #1634964 breaks LXD userspace, in particular the
following commits:

ac7f3f73 (namespace) vfs: Don't modify inodes with a uid or gid unknown to the vfs
ca52383a (namespace) vfs: Don't create inodes with a uid or gid unknown to the vfs

LXD 2.0.6 will include changes to support these kernel changes, but it isn't
available yet on xenial, so for now we just revert these commits.
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

fs/quota/quota.c

@@ -556,7 +556,7 @@ static int quota_setxquota(struct super_block *sb, int type, qid_t id,
 	if (!qid_has_mapping(sb->s_user_ns, qid))
 		return -EINVAL;
 	/* Are we actually setting timer / warning limits for all users? */
-	if (from_kqid(sb->s_user_ns, qid) == 0 &&
+	if (from_kqid(&init_user_ns, qid) == 0 &&
 	    fdq.d_fieldmask & (FS_DQ_WARNS_MASK | FS_DQ_TIMER_MASK)) {
 		struct qc_info qinfo;
 		int ret;