sctp: prevent possible shift-out-of-bounds in sctp_transport_update_rto

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

sctp: prevent possible shift-out-of-bounds in sctp_transport_update_rto

syzbot reported a possible shift-out-of-bounds [1]

Blamed commit added rto_alpha_max and rto_beta_max set to 1000.

It is unclear if some sctp users are setting very large rto_alpha
and/or rto_beta.

In order to prevent user regression, perform the test at run time.

Also add READ_ONCE() annotations as sysctl values can change under us.

[1]

UBSAN: shift-out-of-bounds in net/sctp/transport.c:509:41
shift exponent 64 is too large for 32-bit type 'unsigned int'
CPU: 0 UID: 0 PID: 16704 Comm: syz.2.2320 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x16c/0x1f0 lib/dump_stack.c:120
ubsan_epilogue lib/ubsan.c:233 [inline]
__ubsan_handle_shift_out_of_bounds+0x27f/0x420 lib/ubsan.c:494
sctp_transport_update_rto.cold+0x1c/0x34b net/sctp/transport.c:509
sctp_check_transmitted+0x11c4/0x1c30 net/sctp/outqueue.c:1502
sctp_outq_sack+0x4ef/0x1b20 net/sctp/outqueue.c:1338
sctp_cmd_process_sack net/sctp/sm_sideeffect.c:840 [inline]
sctp_cmd_interpreter net/sctp/sm_sideeffect.c:1372 [inline]