LU-6791 build: Update ZFS/SPL version to 0.6.4.2

LU-6791 build: Update ZFS/SPL version to 0.6.4.2

Updates ZFS and SPL to latest maintence version.  Includes the
following:

Bug Fixes:
* Fix panic due to corrupt nvlist when running utilities
(zfsonlinux/zfs#3335)
* Fix hard lockup due to infinite loop in zfs_zget()
(zfsonlinux/zfs#3349)
* Fix panic on unmount due to iput taskq (zfsonlinux/zfs#3281)
* Improve metadata shrinker performance on pre-3.1 kernels
(zfsonlinux/zfs#3501)
* Linux 4.1 compat: use read_iter() / write_iter()
* Linux 3.12 compat: NUMA-aware per-superblock shrinker
* Fix spurious hung task watchdog stack traces (zfsonlinux/zfs#3402)
* Fix module loading in zfs import systemd service
(zfsonlinux/zfs#3440)
* Fix intermittent libzfs_init() failure to open /dev/zfs
(zfsonlinux/zfs#2556)

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: I053087317ff9e5bedc1671bb46062e96bfe6f074
Reviewed-on: http://review.whamcloud.com/15481
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Jenkins
Reviewed-by: Isaac Huang <he.huang@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>