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)

Change-Id: I68df5dd26d955db860dd016c76109f7d262bad17
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>