net: Add missing annotation for *netlink_seq_start()

Sparse reports a warning at netlink_seq_start()

warning: context imbalance in netlink_seq_start() - wrong count at exit
The root cause is the missing annotation at netlink_seq_start()
Add the missing  __acquires(RCU) annotation

Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>