client: hold FILE_BUFFER ref while waiting for dirty throttle

client: hold FILE_BUFFER ref while waiting for dirty throttle

We may block in the write path because we've reached out dirty data limit.
Hold a reference to the FILE_BUFFER cap during that interval so we don't
lose the cap and put new dirty buffers into the objectcacher out of turn.

(We could also recheck our ability to take the ref after blocking, but I
think this is cleaner.)

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