vos offline: Bring volume back online for -busy

vos offline: Bring volume back online for -busy

vos offline is supposed to bring a volume back online from "busy"
status before exiting, as volumes should not be in "busy" status for
extended periods of time. This was being enforced by required that
-sleep be specified; however, -sleep only results in the volume being
brought back online if a non-zero sleep time was specified. So, make
sure the volume is brought back online if -busy was specified.

Change-Id: I6e605771fcfcabbb09f6dcfd0576c062f64ad93f
Reviewed-on: http://gerrit.openafs.org/5620
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>