No options to tmpfs is valid

If you run a

docker run command with --tmpfs /mountpoint:noexec

Or certain options that get translated into mount options, the mount command can get passed "" for mount data.
So this should be valid.

Signed-off-by: Dan Walsh <dwalsh@redhat.com>

mount/flags.go

@@ -73,6 +73,7 @@ func parseOptions(options string) (int, string) {
 func ParseTmpfsOptions(options string) (int, string, error) {
 	flags, data := parseOptions(options)
 	validFlags := map[string]bool{
+		"":          true,
 		"size":      true,
 		"mode":      true,
 		"uid":       true,