net: nbuf: Use net_nbuf_get_frag() to allocate a fragment

The code used net_nbuf_get_reserve_{rx|tx}_data() function to
allocate a fragment. Instead of that low level function, use
net_nbuf_get_frag() instead. There are few places this is not
possible or is too big change like in few test programs.

Change-Id: Ied7e2b7db352de998b200ffa6ff82471bfa5ebe3
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>