vol: check snprintf return values in namei_ops

vol: check snprintf return values in namei_ops

gcc8 is more aggressive about parsing format strings and computing bounds
on the generated text from functions like snprintf.  In this case it seems best
to detect cases of truncation and error out, rather than trying to increase
stack buffer sizes or switch to asprintf.  These paths should be well-behaved
since they are local to the fileserver, so this is mostly about appeasing the
compiler's -Wformat-truncation checks to allow us to build with --enable-checking.

Change-Id: Id3f15e450c0f03143c0cc7e40186d5944a8fa3b4
Reviewed-on: https://gerrit.openafs.org/13463
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>