aufs: for 3.11, stop loading loop module

Force loading "loop" module by symbol_request() was not a good idea.
Discussing with Florian Klink, aufs decided not to load it.
Instead,
- checks whether the symbol is available or not.
- when it is not available, the adding branch must not be a loopback
  mounted since "loop" module is not loaded.
- when it is available, test whether the adding branch is a loopback
  mounted or not (as usual).

Finally aufs doesn't load "loop" module by itself, but tests by
symbol_get().

Suggested-by: Florian Klink <flokli@flokli.de>
Mail: http://www.mail-archive.com/aufs-users@lists.sourceforge.net/msg04419.html
Signed-off-by: J. R. Okajima <hooanon05@yahoo.co.jp>
Merge remote-tracking branch 'aufs3-standalone/aufs3.11/00base' into aufs3.11/10static
Merge branch 'aufs3.11/10static' into aufs3.11/11proc_map
Merge branch 'aufs3.11/11proc_map' into aufs3.11/18public