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 pre-zerocopy age.

Change-Id: I87c10a2d484b7fe0be370349a2bfeb857ddd74e9
Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-on: http://review.whamcloud.com/12991
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>