io_uring/memmap: explicitly return -EFAULT for mmap on NULL rings

io_uring/memmap: explicitly return -EFAULT for mmap on NULL rings

The later mapping will actually check this too, but in terms of code
clarify, explicitly check for whether or not the rings and sqes are
valid during validation. That makes it explicit that if they are
non-NULL, they are valid and can get mapped.

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