io_uring: fix off-by one bvec index

If the offset equals the bv_len of the first registered bvec, then the
request does not include any of that first bvec. Skip it so that drivers
don't have to deal with a zero length bvec, which was observed to break
NVMe's PRP list creation.

Cc: stable@vger.kernel.org
Fixes: bd11b3a ("io_uring: don't use iov_iter_advance() for fixed buffers")
Signed-off-by: Keith Busch <kbusch@kernel.org>
Link: https://lore.kernel.org/r/20231120221831.2646460-1-kbusch@meta.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: keithbusch

io_uring/rsrc.c

@@ -1258,7 +1258,7 @@ int io_import_fixed(int ddir, struct iov_iter *iter,
 		 */
 		const struct bio_vec *bvec = imu->bvec;
 
-		if (offset <= bvec->bv_len) {
+		if (offset < bvec->bv_len) {
 			/*
 			 * Note, huge pages buffers consists of one large
 			 * bvec entry and should always go this way. The other