afs: Verify osi_UFSOpen worked

  In some builds (UKERNEL) osi_UFSOpen returns a NULL if it runs
into a problem.  On the other builds osi_UFSOpen simply panics.
afs/afs_cell.c was checking for a NULL return but other callers
were not.  Add checking logic to all callers.
  This is a prepartory patch.  A subsequent patch will have
osi_UFSOpen return NULL rather than panic for other builds too.

Change-Id: I3610a57dff59b84fe5ea8b1c862f3192157f255f
Reviewed-on: http://gerrit.openafs.org/11243
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Garrett Wollman <wollman@csail.mit.edu>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: D Brashear <shadow@your-file-system.com>