page_alloc: assert IRQs are enabled in heap alloc/free

page_alloc: assert IRQs are enabled in heap alloc/free

Heap pages can only be safely allocated and freed with interrupts
enabled as they may require a TLB flush which may send IPIs (on x86).

Normally spinlock debugging would catch calls from the incorrect
context, but not from stop_machine_run() action functions as these are
called with spin lock debugging disabled.

Enhance the assertions in alloc_xenheap_pages() and
alloc_domheap_pages() to check interrupts are enabled. For consistency
the same asserts are used when freeing heap pages.

As an exception, when only 1 PCPU is online, allocations are permitted
with interrupts disabled as any TLB flushes would be local only. This
is necessary during early boot.

Signed-off-by: David Vrabel <dvrabel@amazon.co.uk>
Reviewed-by: Jan Beulich <jbeulich@suse.com>