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

0 files changed, 0 insertions, 0 deletions