tcp: Expose tcp_sock_set_keepidle_locked

This is preparation for usage in bpf_setsockopt.

v2:
  - remove redundant EXPORT_SYMBOL (Alexei Starovoitov)

Signed-off-by: Dmitry Yakunin <zeil@yandex-team.ru>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200620153052.9439-2-zeil@yandex-team.ru