sctp: use SCTP_FUTURE_ASSOC for SCTP_PR_SUPPORTED sockopt

Check with SCTP_FUTURE_ASSOC instead in
sctp_set/getsockopt_pr_supported, it's compatible with 0.

It also adjusts some code to keep a same check form as other functions.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>