xsk: Fix backpressure mechanism on Tx

Commit d678cbd2 ("xsk: Fix handling of invalid descriptors in XSK TX
batching API") fixed batch API usage against set of descriptors with
invalid ones but introduced a problem when AF_XDP SW rings are smaller
than HW ones. Mismatch of reported Tx'ed frames between HW generator and
user space app was observed. It turned out that backpressure mechanism
became a bottleneck when the amount of produced descriptors to CQ is
lower than what we grabbed from XSK Tx ring.

Say that 512 entries had been taken from XSK Tx ring but we had only 490
free entries in CQ. Then callsite (ZC driver) will produce only 490
entries onto HW Tx ring but 512 entries will be released from Tx ring
and this is what will be seen by the user space.

In order to fix this case, mix XSK Tx/CQ ring interractions by moving
around internal functions and changing call order:

*  pull out xskq_prod_nb_free() from xskq_prod_reserve_addr_batch()
   up to xsk_tx_peek_release_desc_batch();
** move xskq_cons_release_n() into xskq_cons_read_desc_batch()

After doing so, algorithm can be described as follows:

1. lookup Tx entries
2. use value from 1. to reserve space in CQ (*)
3. Read from Tx ring as much descriptors as value from 2
 3a. release descriptors from XSK Tx ring (**)
4. Finally produce addresses to CQ

Fixes: d678cbd2 ("xsk: Fix handling of invalid descriptors in XSK TX batching API")
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220830121705.8618-1-maciej.fijalkowski@intel.com