net_tstamp: add new flag HWTSTAMP_FLAG_BONDED_PHC_INDEX

Since commit 94dd016a ("bond: pass get_ts_info and SIOC[SG]HWTSTAMP
ioctl to active device") the user could get bond active interface's
PHC index directly. But when there is a failover, the bond active
interface will change, thus the PHC index is also changed. This may
break the user's program if they did not update the PHC timely.

This patch adds a new hwtstamp_config flag HWTSTAMP_FLAG_BONDED_PHC_INDEX.
When the user wants to get the bond active interface's PHC, they need to
add this flag and be aware the PHC index may be changed.

With the new flag. All flag checks in current drivers are removed. Only
the checking in net_hwtstamp_validate() is kept.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>