block: pm: Fix possible unbalanced nr_pending

When someone calls blk_get_request() and blk_put_request()
without invoking blk_execute_rq() in between, nr_pending
will be unbalanced. Fix this by adding a flag, namely the
RQF_PM_ADDED, to track the usage of nr_pending, so that
blk_pm_put_request() won't touch nr_pending if the flag is
not set. Extra care should be given to blk_pm_requeue_request(),
because blk-flush.c inserts flush requests into queue without
calling blk_pm_add_request(), if the flush requests get requeued
by LLD, blk_pm_requeue_request() should anyways decrease
nr_pending in this situation to keep it balanced.

Change-Id: I7baa2049ac532e4fd22a0ce358133ba88266a314
Signed-off-by: Ziqi Chen <ziqichen@codeaurora.org>
Signed-off-by: Can Guo <cang@codeaurora.org>