Fix NFS exports of FUSE file systems for big directories

The FUSE protocol does not require that a directory entry's d_off field
outlive the lifetime of its directory's file handle. Since the NFS
server must reopen the directory on every VOP_READDIR call, that means
it can't pass uio->uio_offset down to the FUSE server. Instead, it must
read the directory from 0 each time. It may need to issue multiple
FUSE_READDIR operations until it finds the d_off field that it's looking
for. That was the intention behind SVN r348209 and r297887, but a logic
bug prevented subsequent FUSE_READDIR operations from ever being issued,
rendering large directories incompletely browseable.

Reviewed by: rmacklem

(cherry picked from commit d088dc76e1a62ecb6c05bd2b14ee48a9f9a7e2bd)

fusefs: optimize NFS readdir for FUSE_NO_OPENDIR_SUPPORT

In its lowest common denominator, FUSE does not require that a directory
entry's d_off field is valid outside of the lifetime of the directory's
FUSE file handle. But since NFS is stateless, it must reopen the
directory on every call to VOP_READDIR. That means reading the
directory all the way from the first entry. Not only does this create
an O(n^2) condition for large directories, but it can also result in
incorrect behavior if either:

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

Handily, for file systems that set FUSE_NO_OPENDIR_SUPPORT d_off is
guaranteed to be valid for the lifetime of the directory entry, there is
no need to read the directory from the start.

Reviewed by: rmacklem

(cherry picked from commit 4a6526d84a56f398732bff491e63aa42f796a27d)

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.

Reviewed by: rmacklem
Differential Revision: https://reviews.freebsd.org/D33726

(cherry picked from commit 00134a07898fa807b8a1fcb2596f0e3644143f69)

fusefs: fix the build without INVARIANTS after 00134a07898

MFC with: 00134a07898fa807b8a1fcb2596f0e3644143f69
Reported by: se

(cherry picked from commit 18ed2ce77a254f365a4687d6afe8eeba2aad2e13)