Linux 5.2 compat: Fix config/kernel-shrink.m4 test failure

"whether ->count_objects callback exists" test failed with
"error: error" message for using an incomplete function shrinker_cb().

This is caused by torvalds/linux@83da1bed86. It's configurable,
but we would want to be able to compile with default kbuild setting.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@osnexus.com>
Closes #8776