io_uring/rw: mark readv/writev as vectored in the opcode definition

This is cleaner than gating on the opcode type, particularly as more
read/write type opcodes may be added.

Then we can use that for the data import, and for __io_read() on
whether or not we need to copy state.

Reviewed-by: Gabriel Krisman Bertazi <krisman@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>