selftests/xsk: print correct error codes when exiting

selftests/xsk: print correct error codes when exiting

Print the correct error codes when exiting the test suite due to some
terminal error. Some of these had a switched sign and some of them
printed zero instead of errno.

Fixes: facb7cb2e909 ("selftests/bpf: Xsk selftests - SKB POLL, NOPOLL")
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/r/20230111093526.11682-5-magnus.karlsson@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>