block: add a flag to identify PM request

Add a flag REQ_PM to identify the request is PM related, such requests
will not change the device request queue's runtime status. It is
intended to be used in driver's runtime PM callback, so that driver can
perform some IO to the device there with the queue's runtime status
unaffected. e.g. in SCSI disk's runtime suspend callback, the disk will
be put into stopped power state, and this require sending a command to
the device. Such command processing should not change the disk's runtime
status.

Change-Id: I522bbee6cc911cefeefba50517ebc283e2838918
Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Git-commit: 66311274691ec65972cad3626057fa8d00c146d8
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
[sthumma@codeaurora.org: resolve trival merge conflicts]
Signed-off-by: Sujit Reddy Thumma <sthumma@codeaurora.org>
Revert "Revert "block: add runtime pm helpers""

This reverts commit f2a8f6b7e81bb94bf08d48f07bb54af0d9a01f7a.
Revert "Revert "block: implement runtime pm strategy""

This reverts commit b376e52e10a364ae17ba6a8da29d7e137a1cfc4a.