nfsd: prepare delegation code for handing out *_ATTRS_DELEG delegations

Add some preparatory code to various functions that handle delegation
types to allow them to handle the OPEN_DELEGATE_*_ATTRS_DELEG constants.
Add helpers for detecting whether it's a read or write deleg, and
whether the attributes are delegated.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>