Revert "blkdev: fix merge_bvec_fn return value checks"

This reverts commit 9f7cdbc3

.

It's causing oopses om dm setups, so revert it until we investigate.

Reported-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Tested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>