LU-9305 osd-zfs: arc_buf could be non-pagesize aligned

LU-9305 osd-zfs: arc_buf could be non-pagesize aligned

ZFS only guarantees PAGE_SIZE alignment to arc_buf_t only when
the block size is not less than (PAGE_SIZE << 2).

The patch for ZFS https://github.com/zfsonlinux/zfs/pull/6084 fixes
the alignment problem, buf Lustre still needs a fix to handle
the problem in case it's running old ZFS release.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I6fd17d7b20499ec0406a3e10cebf6882b92a730f
Reviewed-on: https://review.whamcloud.com/26895
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>