cls/rbd: GroupSnapshotNamespace comparator violates ordering rules

cls/rbd: GroupSnapshotNamespace comparator violates ordering rules

For

  GroupSnapshotNamespace a(1, "group-1", "snap-2");
  GroupSnapshotNamespace b(1, "group-2", "snap-1");

both a < b and b < a evaluate to true.  This violates STL strict weak
ordering requirements which is a problem because GroupSnapshotNamespace
is used as a key in std::map (ictx->snap_ids at least), etc.

Fixes: https://tracker.ceph.com/issues/49792
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 830e72ab9d66c8f5703ea27da5249b02dd16ccd0)