[SCTP]: port randomization

Add port randomization rather than a simple fixed rover
for use with SCTP.  This makes it act similar to TCP, UDP, DCCP
when allocating ports.

No longer need port_alloc_lock as well (suggestion by Brian Haley).

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>