CVE-2026-98586: sctp: hold asoc or transport before mod_timer() in timer handlers

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

sctp: hold asoc or transport before modtimer() in timer handlers

Take the association or transport reference before rearming a timer in the timer handlers.

The existing code calls modtimer() before taking the reference needed by the rearmed timer without holding the sock lock. This creates a race with timer cleanup: if the timer is deleted after modtimer() returns but before the reference is taken, the cleanup path can drop the timer's reference and destroy the transport or association. The timer handler then takes a reference on the already freed object and eventually drops it, causing a refcount underflow.

Hold the object before modtimer() and drop the reference if modtimer() reports that the timer was already pending in timer handlers. Apply the same ordering to the proto-unreachable path, which can rearm a transport timer outside the timer handlers without holding the sock lock.