selftests/bpf: Set errno when urand_spawn() fails

The result of urand_spawn() is checked with ASSERT_OK_PTR, which treats
NULL as success if errno == 0.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230128000650.1516334-8-iii@linux.ibm.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>