sctp: Export sctp_do_peeloff

lookup sctp_association within sctp_do_peeloff() to enable its use outside of
the sctp code with minimal knowledge of the former.

Signed-off-by: Benjamin Poirier <bpoirier@suse.de>
Acked-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>