Fix a couple more unlink()s in vol-salvage.c

Fix a couple more unlink()s in vol-salvage.c

There are a couple more unlink() calls in vol-salvage.c that were using
relative paths. Fix them to use absolute paths and to log a warning if
the unlink() fails.

Reviewed-on: http://gerrit.openafs.org/684
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>