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: 41edf1a ("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>

fs/io_uring.c

@@ -8228,6 +8228,7 @@ static int io_buffer_account_pin(struct io_ring_ctx *ctx, struct page **pages,
 {
 	int i, ret;
 
+	imu->acct_pages = 0;
 	for (i = 0; i < nr_pages; i++) {
 		if (!PageCompound(pages[i])) {
 			imu->acct_pages++;