selftests/bpf: Fix potential unreleased lock

This lock is not released if the program
return at the patched branch.

Signed-off-by: Chengfeng Ye <cyeaa@connect.ust.hk>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210827074140.118671-1-cyeaa@connect.ust.hk