nfsd: ensure new clients break delegations

If nfsd already has an open file that it plans to use for IO from
another, it may not need to do another vfs open, but it still may need
to break any delegations in case the existing opens are for another
client.

Symptoms are that we may incorrectly fail to break a delegation on a
write open from a different client, when the delegation-holding client
already has a write open.

Fixes: 28df3d15 ("nfsd: clients don't need to break their own delegations")
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>