libafs: memset dirHeader->hashTable

libafs: memset dirHeader->hashTable

Clear dirHeader->hashTable via memset instead of via a loop. This is
more efficient, and avoids the loop getting optimized into an unusable
_memset call on recent versions of Solaris Studio when building for
the kernel.

Thanks to Jeff Blaine for reporting the issue with Solaris Studio.

Change-Id: Ibaa5140d510c2df7e1129352a6677594785b42b4
Reviewed-on: http://gerrit.openafs.org/4829
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>