viced: disable rx keepalives during disk io

viced: disable rx keepalives during disk io

when we are going to hit the backend storage, disable keepalives.
the net effect of this is that no idle dead time is needed; instead,
the normal dead time will result in a connection with no activity
simply dying naturally if i/o blocks forever.

it's important that keepalives be enabled during callback breaks,
so that is done.

(cherry picked from commit 05f3a0d1e0359f604cc6162708f3f381eabcd1d7)
Reviewed-on: http://gerrit.openafs.org/6515
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Change-Id: If2ee7f3ad7f2dc835dd350bb9558fde0aa179240
Reviewed-on: http://gerrit.openafs.org/6613
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>