Windows: BUF_HASH use opr_jhash_int2

BUF_HASH takes to 4-byte integers not three and therefore cannot use
the basic opr_jhash which assumes a minimum of three 4-byte integers.

Change-Id: I7f30351025b3e9cd2156f772b0ed550b20964ad7
Reviewed-on: http://gerrit.openafs.org/10624
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>