xen-blkback: fix memory leaks

I've at least identified two possible memory leaks in blkback, both
related to the shutdown path of a VBD:

- blkback doesn't wait for any pending purge work to finish before
  cleaning the list of free_pages. The purge work will call
  put_free_pages and thus we might end up with pages being added to
  the free_pages list after we have emptied it. Fix this by making
  sure there's no pending purge work before exiting
  xen_blkif_schedule, and moving the free_page cleanup code to
  xen_blkif_free.
- blkback doesn't wait for pending requests to end before cleaning
  persistent grants and the list of free_pages. Again this can add
  pages to the free_pages list or persistent grants to the
  persistent_gnts red-black tree. Fixed by moving the persistent
  grants and free_pages cleanup code to xen_blkif_free.

Also, add some checks in xen_blkif_free to make sure we are cleaning
everything.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Tested-by: Matt Rushton <mrushton@amazon.com>
Reviewed-by: Matt Rushton <mrushton@amazon.com>
Cc: Matt Wilson <msw@amazon.com>
Cc: Ian Campbell <Ian.Campbell@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>