Linux 6.5 compat: spl: properly unregister sysctl entries

[arighi]

Motivation and Context

spl fails to properly unload the registered sysctl entries when register_ssyctl_table() is not available in the kernel (>= 6.5).

Corresponding tracking bug in Ubuntu: https://bugs.launchpad.net/bugs/2034510

Description

When register_sysctl_table() is unavailable we fail to properly unregister sysctl entries under "kernel/spl".

This leads to errors like the following when spl is unloaded/reloaded, making impossible to properly reload the spl module:

[ 746.995704] sysctl duplicate entry: /kernel/spl/kmem/slab_kvmem_total

Fix by cleaning up all the sub-entries inside "kernel/spl" when the spl module is unloaded.

How Has This Been Tested?

modprobe -r zfs
modprobe -r spl
modprobe zfs

When the problem happens spl fails to load and we can see the following line in dmesg:

 [  746.995704] sysctl duplicate entry: /kernel/spl/kmem/slab_kvmem_total

Test has been reproduced in a local VM running the latest 6.5 upstream kernel.

Types of changes

• [x ] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• [x ] My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• [ x] I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• [ x] All commit messages are properly formatted and contain Signed-off-by.