Allow negative aio_offset only for the read and write LIO ops on
device nodes.

Otherwise, the current check of aio_offset == -1LL makes it possible
to pass negative file offsets down to the filesystems. This trips
assertions and is even unsafe for e.g. FFS which keeps metadata at
negative offsets.

Reported and tested by: pho
Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D11266