sctp: fix memory leak in sctp_stream_outq_migrate()

sctp: fix memory leak in sctp_stream_outq_migrate()

When sctp_stream_outq_migrate() is called to release stream out resources,
the memory pointed to by prio_head in stream out is not released.

The memory leak information is as follows:
 unreferenced object 0xffff88801fe79f80 (size 64):
   comm "sctp_repo", pid 7957, jiffies 4294951704 (age 36.480s)
   hex dump (first 32 bytes):
     80 9f e7 1f 80 88 ff ff 80 9f e7 1f 80 88 ff ff  ................
     90 9f e7 1f 80 88 ff ff 90 9f e7 1f 80 88 ff ff  ................
   backtrace:
     [<ffffffff81b215c6>] kmalloc_trace+0x26/0x60
     [<ffffffff88ae517c>] sctp_sched_prio_set+0x4cc/0x770
     [<ffffffff88ad64f2>] sctp_stream_init_ext+0xd2/0x1b0
     [<ffffffff88aa2604>] sctp_sendmsg_to_asoc+0x1614/0x1a30
     [<ffffffff88ab7ff1>] sctp_sendmsg+0xda1/0x1ef0
     [<ffffffff87f765ed>] inet_sendmsg+0x9d/0xe0
     [<ffffffff8754b5b3>] sock_sendmsg+0xd3/0x120
     [<ffffffff8755446a>] __sys_sendto+0x23a/0x340
     [<ffffffff87554651>] __x64_sys_sendto+0xe1/0x1b0
     [<ffffffff89978b49>] do_syscall_64+0x39/0xb0
     [<ffffffff89a0008b>] entry_SYSCALL_64_after_hwframe+0x63/0xcd

Link: https://syzkaller.appspot.com/bug?exrid=29c402e56c4760763cc0
Fixes: 637784ade221 ("sctp: introduce priority based stream scheduler")
Reported-by: syzbot+29c402e56c4760763cc0@syzkaller.appspotmail.com
Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Reviewed-by: Xin Long <lucien.xin@gmail.com>
Link: https://lore.kernel.org/r/20221126031720.378562-1-shaozhengchao@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>