net: utils: Helper to check if protocol header fits in fragment

Add utility function that helps to figure out if the
protocol headers can be directly accessed when they fit one
net_buf fragment, or if they need to accessed using various
net_pkt helpers that know about reading data from two
different net_buf's.

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