ANDROID: mnt: Add filesystem private data to mount points

This starts to add private data associated directly
to mount points. The intent is to give filesystems
a sense of where they have come from, as a means of
letting a filesystem take different actions based on
this information.

Bug: 62094374
Bug: 120446149
Bug: 122428178
Change-Id: Ie769d7b3bb2f5972afe05c1bf16cf88c91647ab2
Signed-off-by: Daniel Rosenberg <drosen@google.com>
[astrachan: Folded 89a54ed3bf68 ("ANDROID: mnt: Fix next_descendent")
            into this patch]
[drosen: Folded 138993ea820 ("Android: mnt: Propagate remount
         correctly") into this patch, integrated fs_context things
	 Now has update_mnt_data instead of needing remount2
Signed-off-by: Alistair Strachan <astrachan@google.com>