We need to wait until any currently-running handler has completed.  Fixes an
unplug-time oops reported by "Miles Lane" <miles.lane@gmail.com>.

Cc: "Petko Manolov" <petkan@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>