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()
debugging - kernel resource usage statistics

Signed-off-by: Alexander Boettcher <alexander.boettcher@genode-labs.com>
debugging - dump resource statistics during OOM
debugging - show TRACE_ERROR messages
debugging - perform assertion check ever for this debug branch
debugging - show overmap attempts