CVE-2025-22062: sctp: add mutual exclusion in proc_sctp_do_udp_port()

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

sctp: add mutual exclusion in procsctpdoudpport()

We must serialize calls to sctpudpsockstop() and sctpudpsockstart() or risk a crash as syzbot reported:

Oops: general protection fault, probably for non-canonical address 0xdffffc000000000d: 0000 [#1] SMP KASAN PTI KASAN: null-ptr-deref in range [0x0000000000000068-0x000000000000006f] CPU: 1 UID: 0 PID: 6551 Comm: syz.1.44 Not tainted 6.14.0-syzkaller-g7f2ff7b62617 #0 PREEMPT(full) Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2025 RIP: 0010:kernelsockshutdown+0x47/0x70 net/socket.c:3653 Call Trace: <TASK> udptunnelsockrelease+0x68/0x80 net/ipv4/udptunnelcore.c:181 sctpudpsockstop+0x71/0x160 net/sctp/protocol.c:930 procsctpdoudpport+0x264/0x450 net/sctp/sysctl.c:553 procsyscallhandler+0x3d0/0x5b0 fs/proc/procsysctl.c:601 iterfilesplicewrite+0x91c/0x1150 fs/splice.c:738 dosplicefrom fs/splice.c:935 [inline] directspliceactor+0x18f/0x6c0 fs/splice.c:1158 splicedirecttoactor+0x342/0xa30 fs/splice.c:1102 dosplicedirectactor fs/splice.c:1201 [inline] dosplicedirect+0x174/0x240 fs/splice.c:1227 dosendfile+0xafd/0xe50 fs/readwrite.c:1368 dosyssendfile64 fs/readwrite.c:1429 [inline] sesyssendfile64 fs/readwrite.c:1415 [inline] x64syssendfile64+0x1d8/0x220 fs/readwrite.c:1415 dosyscallx64 arch/x86/entry/syscall64.c:63 [inline]