graid: Avoid tasting devices with small sector sizes

The RAID metadata parsers effectively assume a sector size of 512 bytes
or larger, but md(4) devices can be created with a sector size that's
any power of 2. Add some seatbelts to graid tasting routines to ensure
that the requested sector(s) are large enough for the device to
plausibly contain RAID metadata.

Reported by: syzbot+f43583c9bf8357c8b56f@syzkaller.appspotmail.com
Reported by: syzbot+537dd9f22b91b698e161@syzkaller.appspotmail.com
Reported by: syzbot+51509dd48871c57c6e47@syzkaller.appspotmail.com
Reported by: syzbot+c882a31037ea2a54ff63@syzkaller.appspotmail.com
MFC after: 1 week
Sponsored by: The FreeBSD Foundation