ANDROID: sdcardfs: Move top to its own struct

Move top, and the associated data, to its own struct.
This way, we can properly track refcounts on top
without interfering with the inode's accounting.

Signed-off-by: Daniel Rosenberg <drosen@google.com>
Bug: 38045152
Change-Id: I1968e480d966c3f234800b72e43670ca11e1d3fd