block: Fix use-after-free while iterating over requests

During request completion and if there are no more references,
update the free request with NULL. This could avoid accessing
the already free request from other contexts while iterating
over the requests.

Change-Id: If3593d8397f510821e1d2ca89f5e0af9d19d57e1
Signed-off-by: Pradeep P V K <ppvk@codeaurora.org>