page_pool: fix NULL dereference crash

Check for the existence of page pool params before dereferencing. This can
cause crashes in certain conditions.

Fixes: 35b2e54 ("page_pool: Add callback to init pages when they are
allocated")

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>

net/core/page_pool.c

@@ -213,7 +213,7 @@ static void page_pool_set_pp_info(struct page_pool *pool,
 {
 	page->pp = pool;
 	page->pp_magic |= PP_SIGNATURE;
-	if (pool->p.init_callback)
+	if (pool->p && pool->p.init_callback)
 		pool->p.init_callback(page, pool->p.init_arg);
 }