io_uring: prepare fixed rw for dynanic buffers

With dynamic buffer updates, registered buffers in the table may change
at any moment. First of all we want to prevent future races between
updating and importing (i.e. io_import_fixed()), where the latter one
may happen without uring_lock held, e.g. from io-wq.

Save the first loaded io_mapped_ubuf buffer and reuse.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/21a2302d07766ae956640b6f753292c45200fe8f.1619356238.git.asml.silence@gmail.com


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