block: do not requeue completed request

Verify request is not yet completed before requeueing it,
as requeueing a request ends its tag and sets it to -1 while
it is possible that the request has timed out and it now being
processed for error handling. Since it may be active and processed
in the low lever driver, we mustn't reset its tag.

Change-Id: Ia3e8209959fe377281f28106640a13f10501b47e
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>