selftests/bpf: Reduce more flakyness in sockmap_listen

This patch adds similar retry logic to more places where read() is used, to
reduce flakyness in slow CI environment.

Signed-off-by: Yucong Sun <fallentree@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210825184745.2680830-1-fallentree@fb.com