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 <osalvador@suse.de>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202505060901.Qcs06UoB-lkp@intel.com/
Link: https://lore.kernel.org/r/20250506111012.108743-1-osalvador@suse.de
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

Author: osalvadorvilardaga

mm/mm_init.c

@@ -1786,7 +1786,7 @@ static bool arch_has_descending_max_zone_pfns(void)
 	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();