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_readdir_seq.cpp

@@ -36,6 +36,9 @@ FUSE::ReadDirSeq::operator()(const fuse_req_ctx_t   *ctx_,
 {
   DirInfo *di = DirInfo::from_fh(ffi_->fh);
 
+  if(not di)
+    return -EBADF;
+
   return ::_readdir(cfg.branches,
                     di->fusepath,
                     dirents_);