net: Elaborate output of net_nbuf_print_frags() to be more useful.

Following changes are made:

1. Output using NET_INFO logging level. This function is not part of
automatic logging, and must be called explicitly by a user application.
But if it outputs using NET_DEBUG, then DEBUG level needs to be
enabled, and the output of this function will be drowned in logging
spam. So, let user to enable just INFO level.

2. Show entire structure of the fragment chain, *including* the head
net_buf which holds net_nbuf structure. It is numbered as -1 in the
output to preserve existing order (and not change existing size
calculations).

3. Show owning pool (and its properties, like buffer size/user_data
size) for each buffer.

4. Check for NULL pointer, e.g. for convenience of calling directly
from net_context receive callback (which will be called with NULL
on TCP peer closed connection event).

With these changes, a newcomer from one look at the output of this
function will be able to have a clear basic picture of network
buffer management in Zephyr, and recurring user will be able to
recall any details at once.

Change-Id: I8f9562748329d749f765cc6af7989a448256d7e0
Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>