Declare aio requests on files from local filesystems safe.
Two notes:

• I allow AIO on reclaimed vnodes, since it is deterministically terminated fast.
• devfs mounts are marked as MNT_LOCAL, but device vnodes have type VCHR, so the slow device io is not allowed.

Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D7273