octeontx2-af: Fix multicast/mirror group lock/unlock issue

As per the existing implementation, there exists a race between finding
a multicast/mirror group entry and deleting that entry. The group lock
was taken and released independently by rvu_nix_mcast_find_grp_elem()
function. Which is incorrect and group lock should be taken during the
entire operation of group updation/deletion. This patch fixes the same.

Fixes: 51b2804c

 ("octeontx2-af: Add new mbox to support multicast/mirror offload")
Signed-off-by: Suman Ghosh <sumang@marvell.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>