net: buf: Support copying of user data

This functionality is useful on the following scenario:

1) The first buffer in a net_pkt contains user data which is relevant
   for the (whole) net_pkt.
2) When inserting a new buffer in front of the net_pkt, the (previously)
   first buffer (and its user data) are no longer accessible via
   net_pkt->buffer.
3) net_buf_user_data_copy() allows to simply copy the user data from the
   old to the new first  buffer.

Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>
3 files changed