xdp: xdp_mem_allocator can be NULL in trace_mem_connect().

Since the commit mentioned below __xdp_reg_mem_model() can return a NULL
pointer. This pointer is dereferenced in trace_mem_connect() which leads
to segfault.

The trace points (mem_connect + mem_disconnect) were put in place to
pair connect/disconnect using the IDs. The ID is only assigned if
__xdp_reg_mem_model() does not return NULL. That connect trace point is
of no use if there is no ID.

Skip that connect trace point if xdp_alloc is NULL.

[ Toke Høiland-Jørgensen delivered the reasoning for skipping the trace
  point ]

Fixes: 4a48ef70 ("xdp: Allow registering memory model without rxq reference")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/r/YikmmXsffE+QajTB@linutronix.deSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/xdp.c

@@ -357,7 +357,8 @@ int xdp_rxq_info_reg_mem_model(struct xdp_rxq_info *xdp_rxq,
 	if (IS_ERR(xdp_alloc))
 		return PTR_ERR(xdp_alloc);
 
-	trace_mem_connect(xdp_alloc, xdp_rxq);
+	if (trace_mem_connect_enabled() && xdp_alloc)
+		trace_mem_connect(xdp_alloc, xdp_rxq);
 	return 0;
 }