FUSE: require valid file/dir handle; drop open_files fallback

Several FUSE ops fell back to looking up a FileInfo via
state.open_files when the kernel-provided fh was zero, returning
-ENOENT if the lookup failed. This path papered over handle-tracking
bugs and added cost on every call. Use the handle directly and
return -EBADF when it is invalid - the kernel should always hand us
a valid fh for an open resource.

Also simplify FileInfo::from_fh / DirInfo::from_fh and their to_fh
helpers: callers now handle the null-pointer case explicitly so the
ASSERT_NOT_NULL checks are redundant.

Covers fallocate, fchmod, fchown, fgetattr, flock, ftruncate,
futimens, read, release, releasedir, write, statx, ioctl (file +
dir), readdir (sequential, concurrent-on-read,
concurrent-on-search), and the passthrough fast paths in
create/open.

src/fuse_fchmod.cpp

@@ -43,22 +43,10 @@ FUSE::fchmod(const fuse_req_ctx_t *ctx_,
              cu64                  fh_,
              const mode_t          mode_)
 {
-  u64 fh;
+  FileInfo *fi = FileInfo::from_fh(fh_);
 
-  fh = fh_;
-  if(fh == 0)
-    {
-      state.open_files.cvisit(ctx_->nodeid,
-                              [&](auto &val_)
-                              {
-                                fh = val_.second.fi->to_fh();
-                              });
-    }
-
-  if(fh == 0)
-    return -ENOENT;
-
-  FileInfo *fi = FileInfo::from_fh(fh);
+  if(not fi)
+    return -EBADF;
 
   return ::_fchmod(fi->fd,mode_);
 }