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.
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Closes#15239
adf428cbef