bpf: Add support for absolute value BPF timers

bpf: Add support for absolute value BPF timers

Add a new flag BPF_F_TIMER_ABS that can be passed to bpf_timer_start()
to start an absolute value timer instead of the default relative value.
This makes the timer expire at an exact point in time, instead of a time
with latencies induced by both the BPF and timer subsystems.

Suggested-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Tero Kristo <tero.kristo@linux.intel.com>
Link: https://lore.kernel.org/r/20230302114614.2985072-2-tero.kristo@linux.intel.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>