fs: add uint64 cast to fix Darwin build

Error from Travis CI was:

 +GOOS=darwin
 +GOARCH=amd64
 +go build -tags without_openssl
 # github.com/rfjakob/gocryptfs/internal/fusefrontend
 internal/fusefrontend/fs.go:88:45: cannot use st.Dev (type int32) as type uint64 in argument to openfiletable.NewInumMap

Add uint64 to fix it.

internal/fusefrontend/fs.go

@@ -85,7 +85,7 @@ func NewFS(args Args, c *contentenc.ContentEnc, n nametransform.NameTransformer)
 		args:          args,
 		nameTransform: n,
 		contentEnc:    c,
-		inumMap:       openfiletable.NewInumMap(st.Dev),
+		inumMap:       openfiletable.NewInumMap(uint64(st.Dev)), // cast is needed for Darwin
 	}
 }