spl fails to unregister sysctl entries

[Impact]

On unload the spl module is not properly cleaning up the registered sysctl entries with kernel 6.5 in Mantic:

06:58 ubuntu@mantic$ sudo modprobe spl
modprobe: ERROR: could not insert 'spl': Protocol driver not attached
06:58 ubuntu@mantic$ sudo dmesg | grep duplicate
[ 152.587197] sysctl duplicate entry: /kernel/spl/kmem/slab_kvmem_total

[Test case]

Uninstall zfsutils-linux and try to `modprobe -r zfs; modprobe -r spl`, then `modprobe zfs` again and you should see the error (with a failure to load spl again).

At this point it is possible to load spl again only after a reboot.

[Fix]

Properly cleanup all the registered sysctl entries when the spl module is unloaded.

[Regression potential]

We may experience regressions in systems that are using zfs with the 6.5 kernel.

ProblemType: Bug
DistroRelease: Ubuntu 23.10
Package: zfs-dkms 2.2.0~rc3-0ubuntu3
ProcVersionSignature: User Name 6.5.0-4.4-generic 6.5.0
Uname: Linux 6.5.0-4-generic x86_64
ApportVersion: 2.27.0-0ubuntu2
Architecture: amd64
CasperMD5CheckResult: unknown
CloudArchitecture: x86_64
CloudBuildName: server
CloudID: nocloud
CloudName: unknown
CloudPlatform: nocloud
CloudSerial: 20230823
CloudSubPlatform: config-disk (/dev/vdb)
Date: Wed Sep 6 07:06:33 2023
PackageArchitecture: all
ProcEnviron:
 LANG=C.UTF-8
 PATH=(custom, no user)
 SHELL=/bin/bash
 TERM=xterm-256color
 XDG_RUNTIME_DIR=<set>
SourcePackage: zfs-linux
UpgradeStatus: No upgrade log present (probably fresh install)