io_uring: fix races with buffer table unregister

Fixed buffer table quiesce might unlock ->uring_lock, potentially
letting new requests to be submitted, don't allow those requests to
use the table as they will race with unregistration.

Reported-and-tested-by: van fantasy <g1042620637@gmail.com>
Fixes: bd54b6fe ("io_uring: implement fixed buffers registration similar to fixed files")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>