Linux 6.5 compat: safe cleanup in spl_proc_fini()

If we fail to create a proc entry in spl_proc_init() we may end up
calling unregister_sysctl_table() twice: one in the failure path of
spl_proc_init() and another time during spl_proc_fini().

Avoid the double call to unregister_sysctl_table() and while at it
refactor the code a bit to reduce code duplication.

This was accidentally introduced when the spl code was
updated for Linux 6.5 compatibility.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Closes #15234
Closes #15235