nfsd: grant read delegations to clients holding writes

It's OK to grant a read delegation to a client that holds a write,
as long as it's the only client holding the write.

We originally tried to do this in commit 94415b06 ("nfsd4: a
client's own opens needn't prevent delegations"), which had to be
reverted in commit 6ee65a77

 ("Revert "nfsd4: a client's own
opens needn't prevent delegations"").

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>