net: Ref net_buf using net_nbuf_ref

Using net_buf_ref() technically works but debugging the network buffer
allocations is more difficult if done like that.

Change-Id: Iac81bd3ab95547741d49f32763baaa54e97b4877
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>