xen/page_alloc: Don't hold the heap_lock when clearing PGC_need_scrub

xen/page_alloc: Don't hold the heap_lock when clearing PGC_need_scrub

Currently, the heap_lock is held when clearing PGC_need_scrub in
alloc_heap_pages(). However, this is unnecessary because the only caller
(mark_page_offline()) that can concurrently modify the count_info is
using cmpxchg() in a loop.

Therefore, rework the code to avoid holding the heap_lock and use
test_and_clear_bit() instead.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>