selftests/bpf: Add malloc failure checks in bpf_iter

Since some malloc calls in bpf_iter may at times fail,
this patch adds the appropriate fail checks, and ensures that
any previously allocated resource is appropriately destroyed
before returning the function.

Signed-off-by: Yuran Pereira <yuran.pereira@hotmail.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Kui-Feng Lee <thinker.li@gmail.com>
Link: https://lore.kernel.org/r/DB3PR10MB6835F0ECA792265FA41FC39BE8A3A@DB3PR10MB6835.EURPRD10.PROD.OUTLOOK.COM


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