um: chan_kern: use raw spinlock for irqs_to_free_lock

Since this is called deep in the ARCH=um IRQ infrastructure
it must use a raw spinlock. It's not really part of the
driver, but rather the core UML IRQ code.

Link: https://patch.msgid.link/20250505103358.ae7dc659f8b4.I64ca7aece30e0b4b0b5b35ad89cdd63db197c0ce@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>