kernel - Fix possible race in syncache

kernel - Fix possible race in syncache

* syncache_timer_handler() can block while dropping a syncache entry,
  potentially causing the next pointer it uses in its list iteration
  to become stale.

* Use a list marker to keep tabs on the list position instead of using
  a next pointer.

* Remove critical sections, callouts do not preempt and syncache
  routines can only be entered from protocol threads or callouts.

* Document the callout code as not preempting other threads.

Reported-by: Peter Avalos <pavalos@theshell.com>