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>

xen/common/page_alloc.c

@@ -1032,16 +1032,12 @@ static struct page_info *alloc_heap_pages(
     {
         for ( i = 0; i < (1U << order); i++ )
         {
-            if ( test_bit(_PGC_need_scrub, &pg[i].count_info) )
+            if ( test_and_clear_bit(_PGC_need_scrub, &pg[i].count_info) )
             {
                 if ( !(memflags & MEMF_no_scrub) )
                     scrub_one_page(&pg[i]);
 
                 dirty_cnt++;
-
-                spin_lock(&heap_lock);
-                pg[i].count_info &= ~PGC_need_scrub;
-                spin_unlock(&heap_lock);
             }
             else if ( !(memflags & MEMF_no_scrub) )
                 check_one_page(&pg[i]);