rapidio: use a reference count for struct mport_dma_req

rapidio: use a reference count for struct mport_dma_req

Once the dma request is passed to the DMA engine, the DMA subsystem
would hold a pointer to this structure and could call the completion
callback after do_dma_request() has timed out.

The current code deals with this by putting timed out SYNC requests to a
pending list and freeing them later, when the mport cdev device is
released.  This still does not guarantee that the DMA subsystem is
really done with those transfers, so in theory
dma_xfer_callback/dma_req_free could be called after
mport_cdev_release_dma and could potentially access already freed
memory.

This patch simplifies the current handling by using a kref in the mport
dma request structure, so that it gets freed only when nobody uses it
anymore.

This also simplifies the code a bit, as FAF transfers are now handled in
the same way as SYNC and ASYNC transfers.  There is no need anymore for
the pending list and for the dma workqueue which was used in case of FAF
transfers, so we remove them both.

Link: http://lkml.kernel.org/r/20180405203342.GA16191@nokia.com
Signed-off-by: Ioan Nicu <ioan.nicu.ext@nokia.com>
Acked-by: Alexandre Bounine <alex.bou9@gmail.com>
Cc: Barry Wood <barry.wood@idt.com>
Cc: Matt Porter <mporter@kernel.crashing.org>
Cc: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Logan Gunthorpe <logang@deltatee.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Frank Kunz <frank.kunz@nokia.com>
Cc: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>