ukernel: don't define rdwr env on linux

ukernel: don't define rdwr env on linux

We're not vm rdwr env in ukernel. Don't claim we are, otherwise we
will never push chunks out to the fileserver when our cache is full

Reviewed-on: http://gerrit.openafs.org/5613
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 796de9fb43857acccbd51e2af7c8d67eea90fdc5)

Change-Id: I0256c7bdd8a19a191a5d9981525ee288e614fef0
Reviewed-on: http://gerrit.openafs.org/5708
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>