bpf: Add BPF-helper for accessing CLOCK_TAI

Commit 3dc6ffae2da2 ("timekeeping: Introduce fast accessor to clock tai")
introduced a fast and NMI-safe accessor for CLOCK_TAI. Especially in time
sensitive networks (TSN), where all nodes are synchronized by Precision Time
Protocol (PTP), it's helpful to have the possibility to generate timestamps
based on CLOCK_TAI instead of CLOCK_MONOTONIC. With a BPF helper for TAI in
place, it becomes very convenient to correlate activity across different
machines in the network.

Use cases for such a BPF helper include functionalities such as Tx launch
time (e.g. ETF and TAPRIO Qdiscs) and timestamping.

Note: CLOCK_TAI is nothing new per se, only the NMI-safe variant of it is.

Signed-off-by: Jesper Dangaard Brouer <[email protected]>
[Kurt: Wrote changelog and renamed helper]
Signed-off-by: Kurt Kanzenbach <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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