sh: initialize max_mapnr

sh never initializes max_mapnr which is used by the generic implementation
of pfn_valid().

Initialize max_mapnr with set_max_mapnr() in sh::paging_init().

Link: https://lkml.kernel.org/r/20230214140729.1649961-3-rppt@kernel.org
Fixes: e5080a9 ("mm, arch: add generic implementation of pfn_valid() for FLATMEM")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
Acked-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Greg Ungerer <gerg@linux-m68k.org>
Cc: Rich Felker <dalias@libc.org>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

arch/sh/mm/init.c

@@ -301,6 +301,7 @@ void __init paging_init(void)
 	 */
 	max_low_pfn = max_pfn = memblock_end_of_DRAM() >> PAGE_SHIFT;
 	min_low_pfn = __MEMORY_START >> PAGE_SHIFT;
+	set_max_mapnr(max_low_pfn - min_low_pfn);
 
 	nodes_clear(node_online_map);