linux 6.7 compat: handle superblock shrinker member change

In 6.7 the superblock shrinker member s_shrink has changed from being an
embedded struct to a pointer. Detect this, and don't take a reference if
it already is one.

Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://github.com/sponsors/robn
(cherry picked from commit 18a9185165e2713e690e52347a37de1878e2a9fc)