SCTP: update sctp_getsockopt helpers to allow oversized buffers

	I noted the other day while looking at a bug that was ostensibly
in some perl networking library, that we strictly avoid allowing getsockopt
operations to complete if we pass in oversized buffers.  This seems to make
libraries like Perl::NET malfunction since it seems to allocate oversized
buffers for use in several operations.  It also seems to be out of line with
the way udp, tcp and ip getsockopt routines handle buffer input (since the
*optlen pointer in both an input and an output and gets set to the length
of the data that we copy into the buffer).  This patch brings our getsockopt
helpers into line with other protocols, and allows us to accept oversized
buffers for our getsockopt operations.  Tested by me with good results.

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>