rootfs-block: avoid remount when options don't change

Mounting, unmounting and then mounting a disk partition takes some
time.

On embedded systems such as OLPC XO where we disable fsck and fstab
reading, the root options are not going to change throughout the
mount_root() function, so remounting is time consuming and without
change.

Detect and optimize for this case so that the filesystem is only
mounted once.

modules.d/95rootfs-block/mount-root.sh

@@ -98,20 +98,24 @@ mount_root() {
     # them; rflags is guaranteed to not be empty
     rflags="${rootopts:+"${rootopts},"}${rflags}"
 
-    umount "$NEWROOT"
-
     # backslashes are treated as escape character in fstab
     # esc_root=$(echo ${root#block:} | sed 's,\\,\\\\,g')
     # printf '%s %s %s %s 1 1 \n' "$esc_root" "$NEWROOT" "$rootfs" "$rflags" >/etc/fstab
 
+    ran_fsck=0
     if [ -z "$fastboot" -a "$READONLY" != "yes" ] && ! strstr "${rflags},${rootopts}" _netdev; then
+        umount "$NEWROOT"
         fsck_single "${root#block:}" "$rootfs" "$fsckoptions"
         _ret=$?
         [ $_ret -ne 255 ] && echo $_ret >/run/initramfs/root-fsck
+        ran_fsck=1
     fi
 
+    if [ -n "$rootopts" -o "$ran_fsck" = "1" ]; then
         info "Remounting ${root#block:} with -o ${rflags}"
+        umount "$NEWROOT" &>/dev/null
         mount -t "$rootfs" -o "$rflags" "${root#block:}" "$NEWROOT" 2>&1 | vinfo
+    fi
 
     [ -f "$NEWROOT"/forcefsck ] && rm -f "$NEWROOT"/forcefsck 2>/dev/null
     [ -f "$NEWROOT"/.autofsck ] && rm -f "$NEWROOT"/.autofsck 2>/dev/null