When we snapshot dirty metadata that needs to be written back to the MDS,
include dirty xattr metadata.  Make the capsnap reference the encoded
xattr blob so that it will be written back in the FLUSHSNAP op.

Also fix the capsnap creation guard to include dirty auth or file bits,
not just tests specific to dirty file data or file writes in progress
(this fixes auth metadata writeback).
Signed-off-by: Sage Weil <sage@newdream.net>