mm/mremap: avoid unneeded do_munmap call

When old_len == new_len, do_munmap will return -EINVAL due to len == 0. 
This errno will be simply ignored because of old_len != new_len check.  So
it is unnecessary to call do_munmap when old_len == new_len because
nothing is actually done.

Link: https://lkml.kernel.org/r/20220401081023.37080-1-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/mremap.c

@@ -820,9 +820,9 @@ static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
 			goto out;
 	}
 
-	if (old_len >= new_len) {
+	if (old_len > new_len) {
 		ret = do_munmap(mm, addr+new_len, old_len - new_len, uf_unmap);
-		if (ret && old_len != new_len)
+		if (ret)
 			goto out;
 		old_len = new_len;
 	}