disk: Generalize MD_MAX_DISKS to GRUB_MDRAID_MAX_DISKS

disk: Generalize MD_MAX_DISKS to GRUB_MDRAID_MAX_DISKS

Move the constant from grub-core/osdep/linux/getroot.c to
include/grub/disk.h and then reuse it in place of the
hardcoded 1024 limit in diskfilter.

Fixes: 2a5e3c1f2 (disk/diskfilter: Don't make a RAID array with more than 1024 disks)
Cc: Daniel Axtens <dja@axtens.net>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Julian Andres Klode <julian.klode@canonical.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>