SCTP: Update RCU handling during the ADD-IP case

After learning more about rcu, it looks like the ADD-IP hadling
doesn't need to call call_rcu_bh.  All the rcu critical sections
use rcu_read_lock, so using call_rcu_bh is wrong here.
Now, restore the local_bh_disable() code blocks and use normal
call_rcu() calls.  Also restore the missing return statement.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>