kauth: Remove unused assignments to bp

We never use the character pointer once we've advanced it past the
slash, so don't bother doing the work of advancing it.

Caught by clang-analyzer

Change-Id: I8ea561d6756685a13892f0635fa2e19b3f1233ab
Reviewed-on: http://gerrit.openafs.org/9171
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>