Linux s390x: replace AFS_64BIT_KERNEL with AFS_LINUX_64BIT_KERNEL

Linux s390x: replace AFS_64BIT_KERNEL with AFS_LINUX_64BIT_KERNEL

The s390x param.h file defines AFS_64BIT_KERNEL, but this looks to
be AIX specific, and some Linux headers expect AFS_LINUX_64BIT_KERNEL.
This causes many errors because we end up trying to use struct flock64
instead of struct flock.

Change-Id: I39a4a94fa895143172c1ee75ff71c2015232156f
Reviewed-on: http://gerrit.openafs.org/2131
Tested-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>