net/pkt: Redefine the necessary fifo attribute

commit 79672d16 was missing the fact socket layer is putting net_pkt
into a k_fifo. However, it's on receiving side only: at this point the
k_fifo_put is using the k_work area which is then useless at this
point Thus why it did not break anything, as k_fifo only needs 4 bytes
while the k_work attribute takes 12 bytes.

Thus adding a union on the k_work attribute with another new attribute
that describes the behavior.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>