sctp: add a refcnt in sctp_stream_priorities to avoid a nested loop

In the Linux kernel, the following vulnerability has been resolved:

sctp: add a refcnt in sctp_stream_priorities to avoid a nested loop

With this refcnt added in sctp_stream_priorities, we don't need to traverse all streams to check if the prio is used by other streams when freeing one stream's prio in sctp_sched_prio_free_sid(). This can avoid a nested loop (up to 65535 * 65535), which may cause a stuck as Ying reported:

watchdog: BUG: soft lockup - CPU#23 stuck for 26s! [ksoftirqd/23:136] Call Trace:

sctp_sched_prio_free_sid+0xab/0x100 [sctp] sctp_stream_free_ext+0x64/0xa0 [sctp] sctp_stream_free+0x31/0x50 [sctp] sctp_association_free+0xa5/0x200 [sctp]

Note that it doesn't need to use refcount_t type for this counter, as its accessing is always protected under the sock lock.

v1->v2: - add a check in sctp_sched_prio_set to avoid the possible prio_head refcnt overflow.