fusefs: require FUSE_NO_OPENDIR_SUPPORT for NFS exporting

fusefs: require FUSE_NO_OPENDIR_SUPPORT for NFS exporting

FUSE file systems that do not set FUSE_NO_OPENDIR_SUPPORT do not
guarantee that d_off will be valid after closing and reopening a
directory.  That conflicts with NFS's statelessness, that results in
unresolvable bugs when NFS reads large directories, if:

* The file system _does_ change the d_off field for the last directory
  entry previously returned by VOP_READDIR, or
* The file system deletes the last directory entry previously seen by
  NFS.

Rather than doing a poor job of exporting such file systems, it's better
just to refuse.

Even though this is technically a breaking change, 13.0-RELEASE's
NFS-FUSE support was bad enough that an MFC should be allowed.

MFC after:	3 weeks.
Reviewed by:	rmacklem
Differential Revision: https://reviews.freebsd.org/D33726