dtrace: correct probe disable behaviour for syscalls

dtrace: correct probe disable behaviour for syscalls

Previously, when both entry and return probes were enabled for a
syscall, upon disabling one of them, the function pointer in the
syscall table would already be reset to the default, removing the
interceptor.  This resulted in an inconsistent state when the
2nd probe would get removed, and could cause a nasty race if one
were to try to enable one of the probes in between.

We now only remove the interceptor when we know the last probe
is being disabled.

Orabug: 22352636
Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>
Acked-by: Nick Alcock <nick.alcock@oracle.com>