net: pkt: fix race condition in packet reference counting

It has been observed that some network drivers, f.ex. the SAM E70 GMAC,
call net_pkt_unref from inside the interrupt that signals the successful
transmission of a packet. This conflicts with the net_pkt_unref call
made by ethernet_send after the packet has been given to the driver.

We fix this by using an atomic_t to hold the reference count as there
might be other, difficult to find cases of net_pkt_(un)ref being used
across threads and interrupts.

The name of the element has been changed from "ref" to "atomic_ref" to
cause a compile error when code still has not been converted to use the
atomic_* functions.

Fixes #12708

Signed-off-by: Daniel Glöckner <dg@emlix.com>