ubd: fix missing lock around request issue

We need to hold the device lock (and disable interrupts) while
writing new commands, or we could be interrupted while that
is happening and read invalid requests in the completion path.

Fixes: 4e6da0fe ("um: Convert ubd driver to blk-mq")
Tested-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Jens Axboe <axboe@kernel.dk>