Fix 2.6.35 shrinker callback API change

Fix 2.6.35 shrinker callback API change

As of linux-2.6.35 the shrinker callback API now takes an additional
argument.  The shrinker struct is passed to the callback so that users
can embed the shrinker structure in private data and use container_of()
to access it.  This removes the need to always use global state for the
shrinker.

To handle this we add the SPL_AC_3ARGS_SHRINKER_CALLBACK autoconf
check to properly detect the API.  Then we simply setup a callback
function with the correct number of arguments.  For now we do not make
use of the new 3rd argument.