net: nbuf: Fix the net_nbuf_copy() buf parameter

The first parameter of net_nbuf_copy() must be the head
of the buffer chain i.e., it must contain the user_data
part. If a fragment is given, then we do not know enough
information to allocate the data fragments.

Change-Id: I052b183d8c63d7326b320254f36f00b2fc48b0a0
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>