io_uring: don't quiesce intial files register

There is no reason why we would want to fully quiesce ring on
IORING_REGISTER_FILES, if it's already registered we fail.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/563bb8060bb2d3efbc32fce6101678281c574d2a.1617287883.git.asml.silence@gmail.com


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