inomap: rework logic to efficiently support flags

Adding flags allows to use inomap in reverse mode,
replacing the clunky inoBaseDirIV/inoBaseNameFile
logic that causes problems with high underlying
inode numbers ( https://github.com/rfjakob/gocryptfs/issues/457 )

Microbenchmarks (values below) show that the "SingleDev"
case is now much slower due to an extra map lookup,
but this has no visible effects in ./test.bash results,
so there was no time spent optimizing the case further.

$ go test -bench=.
goos: linux
goarch: amd64
pkg: github.com/rfjakob/gocryptfs/internal/inomap
BenchmarkTranslateSingleDev-4   	18757510	        61.5 ns/op
BenchmarkTranslateManyDevs-4    	18061515	        64.5 ns/op
PASS
ok  	github.com/rfjakob/gocryptfs/internal/inomap	2.467s

1 files changed, 16 insertions, 6 deletions

diff --git a/internal/inomap/qino.go b/internal/inomap/qino.go
index 8f99004..a74a96d 100644
--- a/internal/inomap/qino.go
+++ b/internal/inomap/qino.go
@@ -4,22 +4,32 @@ import (
 	"syscall"
 )
 
+type namespaceData struct {
+	// Stat_t.Dev is uint64 on 32- and 64-bit Linux
+	Dev uint64
+	// Flags acts like an extension of the Dev field.
+	// It is used by reverse mode for virtual files.
+	Flags uint8
+}
+
 // QIno = Qualified Inode number.
 // Uniquely identifies a backing file through the device number,
 // inode number pair.
 type QIno struct {
-	// Stat_t.{Dev,Ino} is uint64 on 32- and 64-bit Linux
-	Dev uint64
+	namespaceData
+	// Stat_t.Ino is uint64 on 32- and 64-bit Linu
 	Ino uint64
 }
 
 // QInoFromStat fills a new QIno struct with the passed Stat_t info.
 func QInoFromStat(st *syscall.Stat_t) QIno {
 	return QIno{
-		// There are some architectures that use 32-bit values here
-		// (darwin, freebsd-32, maybe others). Add and explicit cast to make
-		// this function work everywhere.
-		Dev: uint64(st.Dev),
+		namespaceData: namespaceData{
+			// There are some architectures that use 32-bit values here
+			// (darwin, freebsd-32, maybe others). Add an explicit cast to make
+			// this function work everywhere.
+			Dev: uint64(st.Dev),
+		},
 		Ino: uint64(st.Ino),
 	}
 }