Import a missing piece of commit b8fac8e162eda7e98d from illumos-gate.

This adds an upper bound, dtrace_ustackdepth_max, to the number of frames
traversed when computing the userland stack depth. Some programs - notably
firefox - are otherwise able to trigger an infinite loop in
dtrace_getustack_common(), causing a panic.

MFC after: 1 week