selftests/bpf: Fix sockmap_multi_channels reliability

selftests/bpf: Fix sockmap_multi_channels reliability

Previously I added a FIONREAD test for sockmap, but it can occasionally
fail in CI [1].

The test sends 10 bytes in two segments (2 + 8). For UDP, FIONREAD only
reports the length of the first datagram, not the total queued data.
The original code used recv_timeout() expecting all 10 bytes, but under
high system load, the second datagram may not yet be processed by the
protocol stack, so recv would only return the first 2-byte datagram,
causing a size mismatch failure.

Fix this by receiving exactly the expected bytes (matching FIONREAD) in
the first recv. The remaining datagram is then consumed in a second recv
block, which is only reachable for UDP since TCP's expected already
equals sizeof(buf).

Test:
./test_progs -a sockmap_basic
410/1   sockmap_basic/sockmap create_update_free:OK
...
Summary: 1/35 PASSED, 0 SKIPPED, 0 FAILED

[1] https://github.com/kernel-patches/bpf/actions/runs/22919385910/job/66515395423

Cc: Jiayuan Chen <jiayuan.chen@linux.dev>
Fixes: 17e2ce02bf56 ("selftests/bpf: Add tests for FIONREAD and copied_seq")
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
Link: https://lore.kernel.org/r/20260312072549.6766-1-jiayuan.chen@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>