osd: flush pg on activate _after_ we queue our transaction

osd: flush pg on activate _after_ we queue our transaction

We recently added a flush on activate, but we are still building the
transaction (the caller queues it), so calling osr.flush() here is totally
useless.

Instead, set a flag 'need_flush', and do the flush the next time we receive
some work.

This has the added benefit of doing the flush in the worker thread, outside
of osd_lock.

Signed-off-by: Sage Weil <sage@newdream.net>