nfsd4: clean up nfs4_open_delegation

The nfs4_open_delegation logic is unecessarily baroque.

Also stop pretending we support write delegations in several places.

Some day we will support write delegations, but when that happens adding
back in these flag parameters will be the easy part.  For now they're
just confusing.

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