Revert "block: do not requeue completed request"

This reverts change Ia3e8209959fe377281f28106640a13f10501b47e.

Disabling requeueing of a completed request will cause a request
not to be retried (by requeueing it) in cases where the low-level
driver detected some error but finished processing of the requeust.
Specifically, not retrying the request will cause calls to functions
such as blk_execute_rq to block forever.

Change-Id: Ie0e97cb7560385d48d022dd4ae09f96cfd75b752
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>