Revert "(namespace) dquot: For now explicitly don't support filesystems outside of init_user_ns"

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

This reverts commit c6c8dc16.

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/dquot.c

@@ -2234,11 +2234,6 @@ static int vfs_load_quota_inode(struct inode *inode, int type, int format_id,
 		error = -EINVAL;
 		goto out_fmt;
 	}
-	/* Filesystems outside of init_user_ns not yet supported */
-	if (sb->s_user_ns != &init_user_ns) {
-		error = -EINVAL;
-		goto out_fmt;
-	}
 	/* Usage always has to be set... */
 	if (!(flags & DQUOT_USAGE_ENABLED)) {
 		error = -EINVAL;