zebra: Modify the function to obtain GR client

[Pdoijode]

Modified zebra_gr_enabled_for_vrf() function to obtain the GR client instead of obtaining the client info in the caller function and then passing it to zebra_gr_enabled_for_vrf().

This addresses the following coverity issue:

*** CID 1668072:         Null pointer dereferences  (NULL_RETURNS)
/zebra/zebra_gr.c: 510             in zebra_gr_cleanup_of_non_gr_vrf()
504     		/*
505     		 * If GR is enabled for this VRF, then zebra
506     		 * would have done the stale cleanup when BGP
507     		 * indicated UPDATE_COMPLETE for this VRF for
508     		 * all gr-enabled afi-safis. So skip such VRFs.
509     		 */
>>>     CID 1668072:         Null pointer dereferences  (NULL_RETURNS)
>>>     Dereferencing a pointer that might be "NULL" "client" when calling "zebra_gr_enabled_for_vrf".
510     		if (zebra_gr_enabled_for_vrf(client, vrf->vrf_id))
511     			continue;
512     
513     		for (afi = AFI_IP; afi <= AFI_IP6; afi++) {
514     			table = zvrf->table[afi][SAFI_UNICAST];
515     			if (!table)

[ton31337]

Just to understand: what issue does it solve?

[Pdoijode]

@ton31337 Added the issue to description