Revert "io_uring/rsrc: simplify the bvec iter count calculation"

This reverts commit 2a51c32.

The kernel registered bvecs do use the iov_iter_advance() API, so we
can't rely on this simplification anymore.

Fixes: 27cb27b ("io_uring: add support for kernel registered bvecs")
Reported-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Link: https://lore.kernel.org/r/20250310184825.569371-1-kbusch@meta.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: keithbusch

io_uring/rsrc.c

@@ -1024,7 +1024,7 @@ static int io_import_fixed(int ddir, struct iov_iter *iter,
 	 * and advance us to the beginning.
 	 */
 	offset = buf_addr - imu->ubuf;
-	iov_iter_bvec(iter, ddir, imu->bvec, imu->nr_bvecs, len);
+	iov_iter_bvec(iter, ddir, imu->bvec, imu->nr_bvecs, offset + len);
 
 	if (offset) {
 		/*
@@ -1051,6 +1051,7 @@ static int io_import_fixed(int ddir, struct iov_iter *iter,
 		 * to use the slow iter advance.
 		 */
 		if (offset < bvec->bv_len) {
+			iter->count -= offset;
 			iter->iov_offset = offset;
 		} else if (imu->is_kbuf) {
 			iov_iter_advance(iter, offset);
@@ -1063,6 +1064,7 @@ static int io_import_fixed(int ddir, struct iov_iter *iter,
 
 			iter->bvec += seg_skip;
 			iter->nr_segs -= seg_skip;
+			iter->count -= bvec->bv_len + offset;
 			iter->iov_offset = offset & ((1UL << imu->folio_shift) - 1);
 		}
 	}