ANDROID: incremental-fs: remove index and incomplete dir on umount

Cleanup incremental-fs left overs on umount, otherwise incr-fs will
complain as below:

BUG: Dentry {i=47a,n=.incomplete} still in use [unmount of incremental-fs]

This requires vfs_rmdir() of the special index dir.
Since set_anon_super() was used in incfs_mount_fs() the incfs_kill_sb()
should use kill_anon_super() instead of generic_shutdown_super()
otherwise it will leak the pseudo dev_t that set_anon_super() allocates.

Bug: 211066171

Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org>
Change-Id: I7ea54db63513fc130e1997cbf79121015ee12405