netfilter: ctnetlink: Add missing annotation for ctnetlink_parse_nat_setup()

Sparse reports a warning at ctnetlink_parse_nat_setup()

warning: context imbalance in ctnetlink_parse_nat_setup()
	- unexpected unlock

The root cause is the missing annotation at ctnetlink_parse_nat_setup()
Add the missing __must_hold(RCU) annotation

Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>