cxgb4i : remove spurious use of rcu

As pointed out by the intel guys, there is no need to hold rcu read lock in
cxgbi_inet6addr_handler(), this patch removes it.

Fixes: 759a0cc5 ("cxgb4i: Add ipv6 code to driver, call into libcxgbi ipv6 api")
Signed-off-by: Anish Bhatt <anish@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>