selftests/bpf: test_xsk: Don't exit immediately on allocation failures

selftests/bpf: test_xsk: Don't exit immediately on allocation failures

If any allocation in the pkt_stream_*() helpers fail, exit_with_error() is
called. This terminates the program immediately. It prevents the following
tests from running and isn't compliant with the CI.

Return NULL in case of allocation failure.
Return TEST_FAILURE when something goes wrong in the packet generation.
Clean up the resources if a failure happens between two steps of a test.

Move exit_with_error()'s definition into xskxceiver.c as it isn't used
anywhere else now.

Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Bastien Curutchet (eBPF Foundation) <bastien.curutchet@bootlin.com>
Link: https://lore.kernel.org/r/20251031-xsk-v7-13-39fe486593a3@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>