sctp: Add missing annotation for sctp_transport_walk_start()

Sparse reports a warning at sctp_transport_walk_start()

warning: context imbalance in sctp_transport_walk_start
	- wrong count at exit

The root cause is the missing annotation at sctp_transport_walk_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>