Default number of active kretprobes

[Jim Keniston]

By the way, regarding the problem of underestimating maxactive, we
briefly considered the idea of having a system-wide pool of
kretprobe_instances to draw on in case any particular kretprobe runs
short.  But that adds a little more complexity to the implementation and
to the user interface.  (Presumably, the pool size would be a
configuration option.)

It shouldn't be too tough to prototype that -- probably just adjust
get_free_rp_inst() and recycle_rp_inst().

Jim