block: blk-flush: Add support for Barrier flag

A barrier request is used to control ordering of write requests without
clearing the device's cache. LLD support for barrier is optional. If LLD
doesn't support barrier, flush will be issued instead to insure logical
correctness.
To maintain this fallback flush s/w path and flags are appended.
This patch implements the necessary requests marking in order to support
the barrier feature in the block layer.

This patch implements two major changes required for the barrier support.
(1) A new flush execution-policy is added to support "ordered" requests
    and a fallback , in case barrier is not supported by LLD.
(2) If there is a flush pending in the flush-queue, the received barrier
    is ignored, in order not to miss a demand for an actual flush.

Change-Id: I6072d759e5c3bd983105852d81732e949da3d448
Signed-off-by: Dolev Raviv <draviv@codeaurora.org>