Currently, bonding only obtain the timestamp (ts) information of
the active slave, which is available only for modes 1, 5, and 6.
For other modes, bonding only has software rx timestamping support.

However, some users who use modes such as LACP also want tx timestamp
support. To address this issue, let's check the ts information of each
slave. If all slaves support tx timestamping, we can enable tx
timestamping support for the bond.

Add a note that the get_ts_info may be called with RCU, or rtnl or
reference on the device in ethtool.h>
Suggested-by: Miroslav Lichvar <mlichvar@redhat.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Link: https://lore.kernel.org/r/20230418034841.2566262-1-liuhangbin@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>