mm, thp: drop page_table_lock to uncharge memcg pages

mm->page_table_lock is hotly contested for page fault tests and isn't
necessary to do mem_cgroup_uncharge_page() in do_huge_pmd_wp_page().

Signed-off-by: David Rientjes <[e-mail]>
Cc: KAMEZAWA Hiroyuki <[e-mail]>
Cc: Andrea Arcangeli <[e-mail]>
Acked-by: Johannes Weiner <[e-mail]>
Reviewed-by: Andrea Arcangeli <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/huge_memory.c

···
 	spin_lock(&mm->page_table_lock);
 	put_page(page);
 	if (unlikely(!pmd_same(*pmd, orig_pmd))) {
+		spin_unlock(&mm->page_table_lock);
 		mem_cgroup_uncharge_page(new_page);
 		put_page(new_page);
+		goto out;
 	} else {
 		pmd_t entry;
 		VM_BUG_ON(!PageHead(page));