Switch to jhash for VNODE_HASH

Switch to jhash for VNODE_HASH

Remove the vnodeHashOffset field, as the Jenkins hash will get
a uniform-enough distribution without this extra help.  Per-volume
unique hashing is retained by using the volume ID as the initial
value input to the Jenkins hash.

While here, increase the vnode hash table size from 256 to 2048.

Change-Id: I353dfc8178f13f4e9adcd03a331adf2a7c64a1a9
Reviewed-on: http://gerrit.openafs.org/11666
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>