fs: Catch pioctl failure in mkmount

If the VIOC_FILE_CELL_NAME pioctl fails in fs mkmount, return an
error to the caller, instead of ploughing on with potentially bad
data.

Caught by coverity (#988418)

Reviewed-on: http://gerrit.openafs.org/9267
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 3ad7ba106dea28277b9e1f7a7370ba17b9fb3d17)

Change-Id: I239ff7e567b01ecbec0314ec61f79d2fd0064de7
Reviewed-on: http://gerrit.openafs.org/11008
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>