b=1590, (partial 948):

b=1590, (partial 948):
   Stop flushing on close, and instead flush all outstanding client data
   atomically with the destruction of the final stripe.

   And when flushing for destroy, or other cases of truncate-to-zero, send a
   hint with the AST to indicate that dirty data should be discarded, instead of
   being written back.

   r=zab/phil