kernel: mmu: Fix access to unpacked member inside packed struct

The following warning is triggered by GCC when
-Waddress-of-packed-member is enabled:

/home/carles/src/zephyr/zephyr/kernel/mmu.c: In function
'free_page_frame_list_put':
/home/carles/src/zephyr/zephyr/kernel/mmu.c:383:42: warning: taking
address of packed member of 'struct z_page_frame' may result in an
unaligned pointer value [-Waddress-of-packed-member]
  383 |  sys_slist_append(&free_page_frame_list, &pf->node);

This is due to the fact that sys_snode_t node is an unpacked structure
inside a packed z_page_frame structure, so that the alignment of the
former cannot be ensured if placed inside the latter.

Given that alignment of z_page_frame is ensured by the code, silence the
compiler by going through an intermediate variable.

More info in #16587.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

kernel/mmu.c

@@ -380,7 +380,10 @@ static void free_page_frame_list_put(struct z_page_frame *pf)
 {
 	PF_ASSERT(pf, z_page_frame_is_available(pf),
 		 "unavailable page put on free list");
-	sys_slist_append(&free_page_frame_list, &pf->node);
+	/* The structure is packed, which ensures that this is true */
+	void *node = pf;
+
+	sys_slist_append(&free_page_frame_list, node);
 	z_free_page_count++;
 }