LU-3489 osd-zfs: varsize index support

LU-3489 osd-zfs: varsize index support

LFSCK needs indices with keys/values of different size.
add a support for this to osd-zfs. key/value size can't
vary within a single index object so far.

Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Change-Id: I6faa730ce1f778563c1fcf43d3b50c5b62c899d9
Reviewed-on: http://review.whamcloud.com/7152
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>