vol: use ffs from opr instead of inline

vol: use ffs from opr instead of inline

volume.c defined its own ffs() macro if AFS_HAVE_FFS was not
defined.  Now that ffs() is in opr, just use it from there always.

Change-Id: Ia80a439924541be236b3221b9480143b511c885a
Reviewed-on: http://gerrit.openafs.org/11674
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>