futex: Flag conversion

futex: Flag conversion

Futex has 3 sets of flags:

 - legacy futex op bits
 - futex2 flags
 - internal flags

Add a few helpers to convert from the API flags into the internal
flags.

Signed-off-by: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Reviewed-and-tested-by: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/r/20230807123322.952568452@infradead.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>