Fix ro mount option being passed

"ro" was not parsed out of the string, so it was passed as part of data
to mount().
This would lead to mount() returning an invalid argument code.
Separate out the "ro" option, much like "userxattr", which will allow
the MS-RDONLY mountflag to get set.

Signed-off-by: Ben Foster <bpfoster@gmail.com>
(cherry picked from commit f3daf32)

snapshots/overlay/overlayutils/check.go

@@ -176,6 +176,7 @@ func NeedsUserXAttr(d string) (bool, error) {
 	}
 
 	opts := []string{
+		"ro",
 		fmt.Sprintf("lowerdir=%s:%s,upperdir=%s,workdir=%s", filepath.Join(td, "lower2"), filepath.Join(td, "lower1"), filepath.Join(td, "upper"), filepath.Join(td, "work")),
 		"userxattr",
 	}