mm,mm_init: Mark set_high_memory as __init

set_high_memory() touches arch_zone_lowest_possible_pfn which is
marked as __initdata, which creates a section mismatch.
Since the only user of the function is free_area_init() which is also marked
as __init, mark set_high_memory() as __init as well.

Signed-off-by: Oscar Salvador <[e-mail]>
Reported-by: kernel test robot <[e-mail]>
Closes: https://lore.kernel.org/oe-kbuild-all/[e-mail]/
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mike Rapoport (Microsoft) <[e-mail]>

mm/mm_init.c

···
 	return IS_ENABLED(CONFIG_ARC) && !IS_ENABLED(CONFIG_ARC_HAS_PAE40);
 }
 
-static void set_high_memory(void)
+static void __init set_high_memory(void)
 {
 	phys_addr_t highmem = memblock_end_of_DRAM();