Sometimes we need to know when the progress bar should disappear.

Checking curr >= total wasn't enough since there're cases not met that
condition for the last call.

So add a new ->finish callback to identify this explicitly.  Currently
only GTK frontend needs it.
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1352813436-14173-4-git-send-email-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>