net: Use k_fifo instead of k_work in RX and TX processing

The k_work handler cannot manipulate the used k_work. This means
that it is not easy to cleanup the net_pkt because it contains
k_work in it. Because of this, use k_fifo instead between
RX thread and network driver, and between application and TX
thread.

A echo-server/client run with IPv4 and UDP gave following
results:

Using k_work
------------
TX traffic class statistics:
TC  Priority	Sent pkts	bytes	time
[0] BK (1)	21922		5543071	103 us	[0->41->26->34=101 us]
[1] BE (0)	0		0	-
RX traffic class statistics:
TC  Priority	Recv pkts	bytes	time
[0] BK (0)	0		0	-
[1] BE (0)	21925		6039151	97 us	[0->21->16->37->20=94 us]

Using k_fifo
------------
TX traffic class statistics:
TC  Priority	Sent pkts	bytes	time
[0] BK (1)	15079		3811118	94 us	[0->36->23->32=91 us]
[1] BE (0)	0		0	-
RX traffic class statistics:
TC  Priority	Recv pkts	bytes	time
[0] BK (1)	0		0	-
[1] BE (0)	15073		4150947	79 us	[0->17->12->32->14=75 us]

So using k_fifo gives about 10% better performance with same workload.

Fixes #34690

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
6 files changed