If we have enough memory to allocate a new cap release message, do so, so
that we can send a partial release message immediately.  This keeps us from
making the MDS wait when the cap release it needs is in a partially full
release message.

If we fail because of ENOMEM, oh well, they'll just have to wait a bit
longer.
Signed-off-by: Sage Weil <sage@newdream.net>