ACPI / APEI: Generalise the estatus queue's notify code

ACPI / APEI: Generalise the estatus queue's notify code

Refactor the estatus queue's pool notification routine from
NOTIFY_NMI's handlers. This will allow another notification
method to use the estatus queue without duplicating this code.

Add rcu_read_lock()/rcu_read_unlock() around the list
list_for_each_entry_rcu() walker. These aren't strictly necessary as
the whole nmi_enter/nmi_exit() window is a spooky RCU read-side
critical section.

in_nmi_queue_one_entry() is separate from the rcu-list walker for a
later caller that doesn't need to walk a list.

Signed-off-by: James Morse <james.morse@arm.com>
Reviewed-by: Punit Agrawal <punit.agrawal@arm.com>
Tested-by: Tyler Baicar <tbaicar@codeaurora.org>
[ rjw: Drop unnecessary err variable in two places ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>