This patch add support for SRSI(Surprise Removal Surprise Insertion).

Approach:
---------
Surprise Removal:
-----------------
On surprise removal of the device, gendisk, request queue, device index, sysfs
entries, etc are retained as long as device is in use - mounted filesystem,
device opened by an application, etc. The service thread breaks out of the main
while loop, waits for pci remove to exit, and then waits for device to become
free. When there no holders of the device, service thread cleans up the block
and device related stuff and returns.

Surprise Insertion:
-------------------
No change, this scenario follows the normal pci probe() function flow.
Signed-off-by: Asai Thambi S P <asamymuthupa@micron.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>