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, 8 insertions, 3 deletions

diff --git a/internal/fusefrontend/root_node.go b/internal/fusefrontend/root_node.go
index af4407e..ab0cdcd 100644
--- a/internal/fusefrontend/root_node.go
+++ b/internal/fusefrontend/root_node.go
@@ -61,11 +61,16 @@ func NewRootNode(args Args, c *contentenc.ContentEnc, n *nametransform.NameTrans
 	if len(args.Exclude) > 0 {
 		tlog.Warn.Printf("Forward mode does not support -exclude")
 	}
+	ivLen := nametransform.DirIVLen
+	if args.PlaintextNames {
+		ivLen = 0
+	}
 	rn := &RootNode{
 		args:          args,
 		nameTransform: n,
 		contentEnc:    c,
 		inoMap:        inomap.New(),
+		dirCache:      dirCache{ivLen: ivLen},
 	}
 	// In `-sharedstorage` mode we always set the inode number to zero.
 	// This makes go-fuse generate a new inode number for each lookup.
@@ -122,15 +127,15 @@ func (rn *RootNode) reportMitigatedCorruption(item string) {
 	}
 }
 
-// isFiltered - check if plaintext "path" should be forbidden
+// isFiltered - check if plaintext file "child" should be forbidden
 //
 // Prevents name clashes with internal files when file names are not encrypted
-func (rn *RootNode) isFiltered(path string) bool {
+func (rn *RootNode) isFiltered(child string) bool {
 	if !rn.args.PlaintextNames {
 		return false
 	}
 	// gocryptfs.conf in the root directory is forbidden
-	if path == configfile.ConfDefaultName {
+	if child == configfile.ConfDefaultName {
 		tlog.Info.Printf("The name /%s is reserved when -plaintextnames is used\n",
 			configfile.ConfDefaultName)
 		return true