sctp: fix the proc_handler for sysctl encap_port

proc_dointvec() cannot do min and max check for setting a value
when extra1/extra2 is set, so change it to proc_dointvec_minmax()
for sysctl encap_port.

Fixes: e8a3001c

 ("sctp: add encap_port for netns sock asoc and transport")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>