mdraid1x_linux: Fix gcc10 error -Werror=array-bounds

mdraid1x_linux: Fix gcc10 error -Werror=array-bounds

We bumped into the build error while testing gcc-10 pre-release.

../../grub-core/disk/mdraid1x_linux.c: In function 'grub_mdraid_detect':
../../grub-core/disk/mdraid1x_linux.c:181:15: error: array subscript <unknown> is outside array bounds of 'grub_uint16_t[0]' {aka 'short unsigned int[0]'} [-Werror=array-bounds]
  181 |      (char *) &sb.dev_roles[grub_le_to_cpu32 (sb.dev_number)]
      |               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../grub-core/disk/mdraid1x_linux.c:98:17: note: while referencing 'dev_roles'
   98 |   grub_uint16_t dev_roles[0]; /* Role in array, or 0xffff for a spare, or 0xfffe for faulty.  */
      |                 ^~~~~~~~~
../../grub-core/disk/mdraid1x_linux.c:127:33: note: defined here 'sb'
  127 |       struct grub_raid_super_1x sb;
      |                                 ^~
cc1: all warnings being treated as errors

Apparently gcc issues the warning when trying to access sb.dev_roles
array's member, since it is a zero length array as the last element of
struct grub_raid_super_1x that is allocated sparsely without extra
chunks for the trailing bits, so the warning looks legitimate in this
regard.

As the whole thing here is doing offset computation, it is undue to use
syntax that would imply array member access then take address from it
later. Instead we could accomplish the same thing through basic array
pointer arithmetic to pacify the warning.

Signed-off-by: Michael Chang <mchang@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>