mm/sparse: guard the size of mem_section is power of 2

We usually have this check, while commit 2a3cb8ba ("mm/sparse: delete
old sparse_init and enable new one") missed to take it.

Link: https://lkml.kernel.org/r/20240416012559.4536-1-richard.weiyang@gmail.comSigned-off-by: Wei Yang <richard.weiyang@gmail.com>
Acked-by: Oscar Salvador <osalvador@suse.de>
Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: "Mike Rapoport (IBM)" <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/sparse.c

@@ -560,6 +560,8 @@ void __init sparse_init(void)
 	unsigned long pnum_end, pnum_begin, map_count = 1;
 	int nid_begin;
 
+	/* see include/linux/mmzone.h 'struct mem_section' definition */
+	BUILD_BUG_ON(!is_power_of_2(sizeof(struct mem_section)));
 	memblocks_present();
 
 	pnum_begin = first_present_section_nr();