sh: initialize max_mapnr

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: e5080a967785 ("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>