oslib/linux-blkzoned: fix bogus poiter alignment warning

oslib/linux-blkzoned: fix bogus poiter alignment warning

GCC 10.x reports:

oslib/linux-blkzoned.c: In function ‘blkzoned_report_zones’:
oslib/linux-blkzoned.c:146:18: warning: taking address of packed member of ‘struct blk_zone_report’ may result in an unaligned pointer value [-Waddress-of-packed-member]
  146 |  blkz = (void *) &hdr->zones[0];

which is totally fine, but we can easily avoid this warning by just
setting the 'blkz' pointer to the end of 'hdr'.

Signed-off-by: Jens Axboe <axboe@kernel.dk>