nfsd4: don't close read-write opens too soon

Don't actually close any opens until we don't need them at all.

This means being left with write access when it's not really necessary,
but that's better than putting a file that might still have posix locks
held on it, as we have been.

Reported-by: Toralf Förster <toralf.foerster@gmx.de>
Cc: stable@kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>