Linux 6.7 compat: rework shrinker setup for heap allocations

6.7 changes the shrinker API such that shrinkers must be allocated
dynamically by the kernel. To accomodate this, this commit reworks
spl_register_shrinker() to do something similar against earlier kernels.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://github.com/sponsors/robn
Closes #15681

module/Kbuild.in

@@ -79,6 +79,7 @@ SPL_OBJS := \
 	spl-kstat.o \
 	spl-proc.o \
 	spl-procfs-list.o \
+	spl-shrinker.o \
 	spl-taskq.o \
 	spl-thread.o \
 	spl-trace.o \