net: buf: net_buf_frag_del() had insufficient debugging

In order to see who is freeing the fragment, add function
and line information to net_buf_frag_del() when net_buf
debugging is activated.

Change-Id: I732f579fab2390cb16804cb35b83f46e65fca342
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>