Revert "block: prevent access to NULL pointer in req->part"

This reverts commit f97d4f61.
Although this check prevents NULL reference, it hides the real problem.
Requests that wish to avoid statistics update have to disable the
REQ_IO_STAT flag, otherwise req->part is expected to be initialized.

Change-Id: I680b95ab9aa668612d948770347929ffde30aeab
Signed-off-by: Dolev Raviv <draviv@codeaurora.org>