selftests/xsk: properly handle batch ending in the middle of a packet

selftests/xsk: properly handle batch ending in the middle of a packet

Referenced commit reduced the scope of the variable pkt, so now it has to
be reinitialized via pkt_stream_get_next_rx_pkt(), which also increments
some counters. When the packet is interrupted by the batch ending, pkt
stream therefore proceeds to the next packet, while xsk ring still contains
the previous one, this results in a pkt_nb mismatch.

Decrement the affected counters when packet is interrupted.

Fixes: 8913e653e9b8 ("selftests/xsk: Iterate over all the sockets in the receive pkts function")
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Signed-off-by: Larysa Zaremba <larysa.zaremba@intel.com>
Link: https://lore.kernel.org/r/20260203155103.2305816-2-larysa.zaremba@intel.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>