syscallcompat: also refactor MkdiratUser on GOOS=darwin
syscallcompat: also refactor MkdiratUser on GOOS=darwin

Breakage was:

+GOOS=darwin
+GOARCH=amd64
+go build -tags without_openssl
internal/fusefrontend/node_dir_ops.go:45:34: cannot use context (type *fuse.Context) as type *fuse.Caller in argument to syscallcompat.MkdiratUser
internal/fusefrontend/node_dir_ops.go:83:35: cannot use context (type *fuse.Context) as type *fuse.Caller in argument to syscallcompat.MkdiratUser

rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs