microblaze: tag highmem_setup() with __meminit

With commit a0cd7a7c4b ("mm: simplify free_highmem_page() and
free_reserved_page()") the kernel test robot complains about a warning:

WARNING: modpost: vmlinux.o(.text.unlikely+0x23ac): Section mismatch in
  reference from the function highmem_setup() to the function
  .meminit.text:memblock_is_reserved()

This has been broken ever since microblaze added highmem support,
because memblock_is_reserved() was already tagged with "__init" back then -
most probably the function always got inlined, so we never stumbled over
it.

We need __meminit because __init_memblock defaults to that without
CONFIG_ARCH_KEEP_MEMBLOCK" and __init_memblock is not used outside memblock
code.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 2f2f371f89 ("microblaze: Highmem support")
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Arvind Sankar <nivedita@alum.mit.edu>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Link: https://lore.kernel.org/r/20210301114749.47914-1-david@redhat.com
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/microblaze/mm/init.c

@@ -52,7 +52,7 @@ static void __init highmem_init(void)
 	pkmap_page_table = virt_to_kpte(PKMAP_BASE);
 }
 
-static void highmem_setup(void)
+static void __meminit highmem_setup(void)
 {
 	unsigned long pfn;