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.

Reviewed-on: http://gerrit.openafs.org/4829
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit f091ace32e3045da396d577055dafd67888ff7ea)

Change-Id: Ia098730c3e83429ce4f886b1427159d13eff4c4e
Reviewed-on: http://gerrit.openafs.org/6414
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>