Disable preemption for rcu::call during revoke

Workaround to avoid situation that in Rcu::update() the curr.head pointer
after some while does not become null and the next list entries increases
until kernel stops with out of memory.

Seems to be problem when Rcu_list::enqueue and Rcu_list::clear are used
concurrently (one CPU, preemption enabled during Pd::revoke)

lapic IRQ -> handler calls Rcu::update() -> Rcu_list::append() -> Rcu_list::clear()

user thread -> revoke -> Rcu::call -> Rcu_list::enqueue()