net: nbuf: Create net_nbuf_frag_del() for tracking allocations

Create net_nbuf_frag_del() so that we can track net_buf frees.
If CONFIG_NET_DEBUG_NET_BUF is not defined, then this new function
will call net_buf_frag_del() directly, and if not, then it will
track memory usage.

Change-Id: I5f382436cebc71fdaf12baf7bf964fb63bee7aca
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>