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.

Change-Id: I1a7bfe0bc62a092ca7dd6dbc4710f1b8254ca9a1
Reviewed-on: http://gerrit.openafs.org/6515
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>