io_uring/rw: compile out compat param passing

Even when COMPAT is compiled out, we still have to pass
ctx->compat to __import_iovec(). Replace the read with an indirection
with a constant when the kernel doesn't support compat.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Anuj Gupta <anuj20.g@samsung.com>
Link: https://lore.kernel.org/r/2819df9c8533c36b46d7baccbb317a0ec89da6cd.1740400452.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>