nfsd: have nfsd4_deleg_getattr_conflict pass back write deleg pointer

Currently we pass back the size and whether it has been modified, but
those just mirror values tracked inside the delegation. In a later
patch, we'll need to get at the timestamps in the delegation too, so
just pass back a reference to the write delegation, and use that to
properly override values in the iattr.

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