fusefrontend: implement recursive diriv caching

The new contrib/maxlen.bash showed that we have exponential
runtime with respect to directory depth.

The new recursive diriv caching is a lot smarter as it caches
intermediate lookups. maxlen.bash now completes in a few seconds.

xfstests results same as
https://github.com/rfjakob/fuse-xfstests/blob/2d158e4c82be85c15269af77498e353f928f4fab/screenlog.0 :

  Failures: generic/035 generic/062 generic/080 generic/093 generic/099 generic/215 generic/285 generic/319 generic/426 generic/444 generic/467 generic/477 generic/523
  Failed 13 of 580 tests

benchmark.bash results are identical:

  $ ./benchmark.bash
  Testing gocryptfs at /tmp/benchmark.bash.BdQ: gocryptfs v2.0.1-17-g6b09bc0; go-fuse v2.1.1-0.20210611132105-24a1dfe6b4f8; 2021-06-25 go1.16.5 linux/amd64
  /tmp/benchmark.bash.BdQ.mnt is a mountpoint
  WRITE: 262144000 bytes (262 MB, 250 MiB) copied, 0,4821 s, 544 MB/s
  READ:  262144000 bytes (262 MB, 250 MiB) copied, 0,266061 s, 985 MB/s
  UNTAR: 8,280
  MD5:   4,564
  LS:    1,745
  RM:    2,244

1 files changed, 1 insertions, 1 deletions

diff --git a/internal/fusefrontend/node_open_create.go b/internal/fusefrontend/node_open_create.go
index 8b31932..6385bc1 100644
--- a/internal/fusefrontend/node_open_create.go
+++ b/internal/fusefrontend/node_open_create.go
@@ -16,7 +16,7 @@ import (
 //
 // Symlink-safe through Openat().
 func (n *Node) Open(ctx context.Context, flags uint32) (fh fs.FileHandle, fuseFlags uint32, errno syscall.Errno) {
-	dirfd, cName, errno := n.prepareAtSyscall("")
+	dirfd, cName, errno := n.prepareAtSyscallMyself()
 	if errno != 0 {
 		return
 	}