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

We usually have this check, while commit 2a3cb8baef71 ("mm/sparse: delete
old sparse_init and enable new one") missed to take it.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Wei Yang <[e-mail]>
Acked-by: Oscar Salvador <[e-mail]>
Reviewed-by: Pasha Tatashin <[e-mail]>
Cc: "Mike Rapoport (IBM)" <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/sparse.c

···
 	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();