xen-blkback: replace work_pending with work_busy in purge_persistent_gnt()

xen-blkback: replace work_pending with work_busy in purge_persistent_gnt()

commit 53bc7dc004fecf39e0ba70f2f8d120a1444315d3 upstream.

The BUG_ON() in purge_persistent_gnt() will be triggered when previous purge
work haven't finished.

There is a work_pending() before this BUG_ON, but it doesn't account if the work
is still currently running.

Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Bob Liu <bob.liu@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>