futex: Clarify FUTEX2 flags

futex: Clarify FUTEX2 flags

sys_futex_waitv() is part of the futex2 series (the first and only so
far) of syscalls and has a flags field per futex (as opposed to flags
being encoded in the futex op).

This new flags field has a new namespace, which unfortunately isn't
super explicit. Notably it currently takes FUTEX_32 and
FUTEX_PRIVATE_FLAG.

Introduce the FUTEX2 namespace to clarify this

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/20230807123322.814039156@infradead.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>