nfsd4: teach encoders to handle reserve_space failures

We've tried to prevent running out of space with COMPOUND_SLACK_SPACE
and special checking in those operations (getattr) whose result can vary
enormously.

However:
	- COMPOUND_SLACK_SPACE may be difficult to maintain as we add
	  more protocol.
	- BUG_ON or page faulting on failure seems overly fragile.
	- Especially in the 4.1 case, we prefer not to fail compounds
	  just because the returned result came *close* to session
	  limits.  (Though perfect enforcement here may be difficult.)
	- I'd prefer encoding to be uniform for all encoders instead of
	  having special exceptions for encoders containing, for
	  example, attributes.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>