LU-4820 osd: drop memcpy in zfs osd

LU-4820 osd: drop memcpy in zfs osd

dmu_read() was called from osd_read_prep() copying from
ARC bufs into the same ARC bufs. seem to be the remainings
of per-zerocopy age.

Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Change-Id: I0f3657c360d8541d7c3c6e8e32eac78bc5702b42
Reviewed-on: http://review.whamcloud.com/9760
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Isaac Huang <he.huang@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>