Merge branch 'add-option-to-provide-opt_id-value-via-cmsg'

Vadim Fedorenko says:

====================
Add option to provide OPT_ID value via cmsg

SOF_TIMESTAMPING_OPT_ID socket option flag gives a way to correlate TX
timestamps and packets sent via socket. Unfortunately, there is no way
to reliably predict socket timestamp ID value in case of error returned
by sendmsg. For UDP sockets it's impossible because of lockless
nature of UDP transmit, several threads may send packets in parallel. In
case of RAW sockets MSG_MORE option makes things complicated. More
details are in the conversation [1].
This patch adds new control message type to give user-space
software an opportunity to control the mapping between packets and
values by providing ID with each sendmsg.

The first patch in the series adds all needed definitions and implements
the function for UDP sockets. The explicit check of socket's type is not
added because subsequent patches in the series will add support for other
types of sockets. The documentation is also included into the first
patch.

Patch 2/4 adds support for TCP sockets. This part is simple and straight
forward.

Patch 3/4 adds support for RAW sockets. It's a bit tricky because
sock_tx_timestamp functions has to be refactored to receive full socket
cookie information to fill in ID. The commit b534dc46c8ae ("net_tstamp:
add SOF_TIMESTAMPING_OPT_ID_TCP") did the conversion of sk_tsflags to
u32 but sock_tx_timestamp functions were not converted and still receive
16b flags. It wasn't a problem because SOF_TIMESTAMPING_OPT_ID_TCP was
not checked in these functions, that's why no backporting is needed.

Patch 4/4 adds selftests for new feature.

[1] https://lore.kernel.org/netdev/CALCETrU0jB+kg0mhV6A8mrHfTE1D1pr1SD_B9Eaa9aDPfgHdtA@mail.gmail.com/
====================

Link: https://patch.msgid.link/20241001125716.2832769-1-vadfed@meta.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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