librbd: Add a paramter:purge_on_error in ImageCtx::invalidate_cache().

librbd: Add a paramter:purge_on_error in ImageCtx::invalidate_cache().

If bh_write met error, it will try again. For closing image, if met this
issue, it will trigger a assert:
>>2015-02-03 15:22:49.198292 7ff62d537800 -1 osdc/ObjectCacher.cc: In
function 'ObjectCacher::~ObjectCacher()' thread 7ff62d537800 time
>>2015-02-03 15:22:49.195927osdc/ObjectCacher.cc: 551: FAILED
>>assert(i->empty())

Now add purge_on_error, when shutdown_cache it set true.
In ImageCtx::invalidate_cache, if met error and purge_on_error is true,
purge the dirty bh.

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>