io_uring: add IOSQE_BUFFER_SELECT support for IORING_OP_READV

This adds support for the vectored read. This is limited to supporting
just 1 segment in the iov, and is provided just for convenience for
applications that use IORING_OP_READV already.

The iov helpers will be used for IORING_OP_RECVMSG as well.

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