sctp: Add missing annotation for sctp_err_finish()

Sparse reports a warning at sctp_err_finish()
warning: context imbalance in sctp_err_finish() - unexpected unlock

The root cause is a missing annotation at sctp_err_finish()
Add the missing  __releases(&((__sk)->sk_lock.slock)) annotation

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