core/cgroup: allow setting oomd xattrs from user manager

The extended attributes 'user.oomd_avoid' and 'user.oomd_omit' are used
to store the setting of the ManagedOOMPrefernce property. Since these
are user extended attributes, the access permissions are defined by the
file permissions, and there is no need to guard this from user managers.

src/core/cgroup.c

@@ -783,6 +783,9 @@ static void cgroup_xattr_apply(Unit *u) {
 
         assert(u);
 
+        /* The 'user.*' xattrs can be set from a user manager. */
+        cgroup_oomd_xattr_apply(u, u->cgroup_path);
+
         if (!MANAGER_IS_SYSTEM(u->manager))
                 return;
 
@@ -809,8 +812,6 @@ static void cgroup_xattr_apply(Unit *u) {
                 else
                         unit_remove_xattr_graceful(u, NULL, xn);
         }
-
-        cgroup_oomd_xattr_apply(u, u->cgroup_path);
 }
 
 static int lookup_block_device(const char *p, dev_t *ret) {