If a request fails, we need to ensure we set REQ_F_FAIL_LINK on it if
REQ_F_LINK is set. Any failure in the chain should break the chain.

We were missing a few spots where this should be done. It might be nice
to generalize this somewhat at some point, as long as we factor in the
fact that failure looks different for each request type.
Signed-off-by: Jens Axboe <axboe@kernel.dk>