net/sctp: use in_compat_syscall for sctp_getsockopt_connectx3

SCTP unfortunately has a different ABI for SCTP_SOCKOPT_CONNECTX3 for
32-bit and 64-bit callers.  Use in_compat_syscall to correctly
distinguish them on all architectures.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Vlad Yasevich <vyasevich@gmail.com>
Cc: Neil Horman <nhorman@tuxdriver.com>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>