Revert "LU-4820 osd: drop memcpy in zfs osd"

This caused review-zfs sanity test_44 to fail in all test cases. 

This reverts commit 1249edcd71e6a44f92aba1482201b30696e85d0d.

Change-Id: I972c4c68ee67443c999ce74fda6f6960b0e4b30d
Reviewed-on: http://review.whamcloud.com/12990
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Tested-by: Oleg Drokin <oleg.drokin@intel.com>