viced/callback.c: Ignore dump write errors harder

When writing a callback state dump, test the return values from
write(2), but don't do anything based on the test.  This avoids
compiler warnings when building on Ubuntu 12.10, with gcc 4.7.2 and
eglibc 2.15-0ubuntu20.1.  This adds a new macro, WriteBytes(), which
handles writing a requested number of bytes to a file and ignoring
errors.

Change-Id: Icead6b25a290cea09a91184dc12b7449cfaaf6f7
Reviewed-on: http://gerrit.openafs.org/9991
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>