net: nbuf: Add function to split a data fragment

Add net_nbuf_split() function that can be used to split
an existing net_buf fragment into two arbitrary length pieces,
and return the two new fragments to the caller. The data from
the original fragment is copied into these two new fragments.

Existing fragment is not modified.

Change-Id: I463e675232c6e19c2a42929f480893a6d1265873
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>