SCTP: Fix sctp_getsockopt_get_peer_addrs

	This is the split out of the patch that we agreed I should split
out from my last patch.  It changes space_left to be computed in the same
way the to variable is.  I know we talked about changing space_left to an
int, but I think size_t is more appropriate, since we should never have
negative space in our buffer, and computing using offsetof means space_left
should now never drop below zero.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Sridhar Samudrala <sri@us.ibm.com>
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>