net: buf: Simplify querying maximum len

net_buf_max_len() provides the maximum number of bytes which can be put
behind the data pointer. This provides a save alternative to using the
size field of the net_buf structure directly, which does not take the
reserved bytes (headroom) into account.

This commit also replaces the usage of the size field in places where
size got used directly. Code has not been adjusted when it is easy to
recognise that the buffer does not have any reserved bytes, which is the
case after allocation or reset. Same goes for the faulty usage by
net_pkt as exposed by the last commit and begin fixed by a separate
commit.

Even though it would be cleaner, I decided to not rename the size field
to e.g. __buf_size in order to keep the amount of code changes low.

Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>