libcutils: fix rw FS remount check issue

In some condition, some of FS would set disable_roll_forward option.
In this case, when we do power off or reboot, the power down progress
would stuck at android_reboot for 360 sec.
The reason is, remount_ro_done function check key word "rw" to determine
whether all the read-write FS has been remounted as read-only FS.
Unfortunately, it takes disable_roll_forward as a rw flag too by
mistake, so the check would always fail, until reach 360 sec threshold.

The patch would match "rw," from mount options, this can avoid the issue
successfully.

Change-Id: I409f9dc88c9ee5cf049615ea9dcccaf195b6bc5a
Signed-off-by: Hong-Mei Li <a21834@motorola.com>