ACPI / APEI: Add missing synchronize_rcu() on NOTIFY_SCI removal

When removing a GHES device notified by SCI, list_del_rcu() is used,
ghes_remove() should call synchronize_rcu() before it goes on to call
kfree(ghes), otherwise concurrent RCU readers may still hold this list
entry after it has been freed.

Signed-off-by: James Morse <james.morse@arm.com>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Fixes: 81e88fdc (ACPI, APEI, Generic Hardware Error Source POLL/IRQ/NMI notification type support)
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>