ANDROID: sdcardfs: Protect set_top

If the top is changed while we're attempting to use it, it's
possible that the reference will be put while we are in the
process of grabbing a reference.

Now we grab a spinlock to protect grabbing our reference count.

Additionally, we now set the inode_info's top value to point to
it's own data when initializing, which makes tracking changes
easier.

Change-Id: If15748c786ce4c0480ab8c5051a92523aff284d2
Signed-off-by: Daniel Rosenberg <drosen@google.com>