ospf6d: ospf6d is crashing upon receiving duplicated Grace LSA.

ospf6d: ospf6d is crashing upon receiving duplicated Grace LSA.

Description:
	When grace lsa received, DUT is adding
	the copy of the  lsas  to all nbrs retransmission  list as part of
	flooding procedure and subsequently incrementing the rmt counter in
	the original the LSA. This counter is supposed to be decremented
	when ack is received by nbr and the lsa  will be removed from retransmission list.

	But in our current scenario,
	Step-1:
		When GR helper is disabled, if DUT receives the grace lsa
		it adds the lsa copy to nbrs retransmission list but original
		LSA will be discarded since GR helper disabled.
	Step-2:
		GR helper enabled and DUT receives the grace lsa, as part
		of flooding process all nbrs have same copy of lsa in their
		corresponding rmt list which was added in step -1 due to this
		the corresponding rmt counter in the original lsa is not getting
		incremented.
	Step-3:
		If the same copy of the grace lsa received by DUT, It considers
		as implicit ack from nbr if the same copy of the lsa exits in its
		rmt list and subsequently  decrement the rmt counter.
	Since counter is zero (because of step-1 and 2) , it is asserting while decrement.

Signed-off-by: Rajesh Girada <rgirada@vmware.com>
(cherry picked from commit a60eab9e109fd2625dc3f941d7d9be2b6312f593)