libata: add refcounting to ata_host

After commit 9a6d6a2d ("ata: make ata port as parent device of scsi
host") manual driver unbind/remove causes use-after-free.

Unbind unconditionally invokes devres_release_all() which calls
ata_host_release() and frees ata_host/ata_port memory while it is still
being referenced as a parent of SCSI host. When SCSI host is finally
released scsi_host_dev_release() calls put_device(parent) and accesses
freed ata_port memory.

Add reference counting to make sure that ata_host lives long enough.

Bug report: https://lkml.org/lkml/2017/11/1/945
Fixes: 9a6d6a2d

 ("ata: make ata port as parent device of scsi host")
Cc: Tejun Heo <tj@kernel.org>
Cc: Lin Ming <minggr@gmail.com>
Cc: linux-ide@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Taras Kondratiuk <takondra@cisco.com>
Signed-off-by: Tejun Heo <tj@kernel.org>