We can drop caps with an mds request.  Ensure we only drop unused AND
clean caps, since the MDS doesn't support cap writeback in that context,
nor do we track it.  If caps are dirty, and the MDS needs them back, we
it will revoke and we will flush in the normal fashion.

This fixes a possibly loss of metadata.
Signed-off-by: NSage Weil <sage@newdream.net>