io_uring: remove duplicated member check for io_msg_ring_prep()

Julia and the kernel test robot report that the prep handling for this
command inadvertently checks one field twice:

fs/io_uring.c:4338:42-56: duplicated argument to && or ||

Get rid of it.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Fixes: 4f57f06c ("io_uring: add support for IORING_OP_MSG_RING command")
Signed-off-by: Jens Axboe <axboe@kernel.dk>