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.

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>
(cherry picked from commit 8632f23d6718a3cd621791e82d1cf6ead8690978)

Change-Id: Ie8f9005ad9cf7cdfd3eb472e01a6fdbde5b7e57e
Reviewed-on: https://gerrit.openafs.org/13732
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>