volume_inline.h: Down with assert, again

Commit 34767c6a0f914960c9a1efabe69dd9c312a2b400 replaced all assert
calls in this file with osi_Assert (now opr_Assert), but shortly
thereafter, commit db6ee95864a8fc5f33b7e95c19c8ff5058d37e92 added
VTimedWaitStateChange_r with two new assert calls.  These are
precarious in a public header; fix them to opr_Assert like the ones in
VWaitStateChange_r.

Change-Id: If2055355b45a09940d69dace59ec18bd6a0cc6fa
Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-on: http://gerrit.openafs.org/10094
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>