Merge branch 'export send_recv_data'

Geliang Tang says:

====================
v5:
 - address Martin's comments for v4 (thanks).
 - update patch 2, use 'return err' instead of 'return -1/0'.
 - drop patch 3 in v4.

v4:
 - fix a bug in v3, it should be 'if (err)', not 'if (!err)'.
 - move "selftests/bpf: Use log_err in network_helpers" out of this
   series.

v3:
 - add two more patches.
 - use log_err instead of ASSERT in v3.
 - let send_recv_data return int as Martin suggested.

v2:

Address Martin's comments for v1 (thanks.)
 - drop patch 1, "export send_byte helper".
 - drop "WRITE_ONCE(arg.stop, 0)".
 - rebased.

send_recv_data will be re-used in MPTCP bpf tests, but not included
in this set because it depends on other patches that have not been
in the bpf-next yet. It will be sent as another set soon.
====================

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