selftests/bpf: Fix error checks of mptcp open_and_load

Return libbpf_get_error(), instead of -EIO, for the error from
mptcp_sock__open_and_load().

Load success means prog_fd and map_fd are always valid. So drop these
unneeded ASSERT_GE checks for them in mptcp run_test().

Acked-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Link: https://lore.kernel.org/r/db5fcb93293df9ab173edcbaf8252465b80da6f2.1692147782.git.geliang.tang@suse.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>