io_uring: fix misaccounting fix buf pinned pages

As Andres reports "... io_sqe_buffer_register() doesn't initialize imu.
io_buffer_account_pin() does imu->acct_pages++, before calling
io_account_mem(ctx, imu->acct_pages).", leading to evevntual -ENOMEM.

Initialise the field.

Reported-by: Andres Freund <andres@anarazel.de>
Fixes: 41edf1a5

 ("io_uring: keep table of pointers to ubufs")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/438a6f46739ae5e05d9c75a0c8fa235320ff367c.1622285901.git.asml.silence@gmail.com


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