net: nbuf: Fix net_nbuf_copy() so that original nbuf is not modified

It makes no sense to modify the original net_buf so change the
copy function not to touch the original buffer.

Change-Id: I5d22445ce50cee62994c36567f0e995a500cb89d
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>