selftests/bpf: Fix memory leak in msg_alloc_iov

In the msg_alloc_iov function, the iov pointer is only assigned to
msg->msg_iov after all memory allocations complete successfully.
Therefore, when a calloc failure triggers the unwind_iov cleanup branch,
we should use the local variable iov instead of msg->msg_iov.

Fixes: 753fb2ee0934 ("bpf: sockmap, add msg_peek tests to test_sockmap")
Signed-off-by: Feng Yang <yangfeng@kylinos.cn>
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20260707081434.539327-1-yangfeng59949@163.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>