Bonding: return HWTSTAMP_FLAG_BONDED_PHC_INDEX to notify user space

If the userspace program is distributed in binary form (distro package),
there is no way to know on which kernel versions it will run.

Let's only check if the flag was set when do SIOCSHWTSTAMP. And return
hwtstamp_config with flag HWTSTAMP_FLAG_BONDED_PHC_INDEX to notify
userspace whether the new feature is supported or not.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Fixes: 085d6100 ("Bonding: force user to add HWTSTAMP_FLAG_BONDED_PHC_INDEX when get/set HWTSTAMP")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>