Fix a backwards comparison in the code to dump a DTrace debug buffer.

PR: 217739
MFC after: 1 week

Correct a series of errors in the hand-rolled locking for drace_debug.c:

- Use spinlock_enter()/spinlock_exit() to prevent a thread holding a
debug lock from being preempted to prevent other threa

Correct a series of errors in the hand-rolled locking for drace_debug.c:

- Use spinlock_enter()/spinlock_exit() to prevent a thread holding a
debug lock from being preempted to prevent other threads waiting
on that lock from starvation.

- Handle the possibility of CPU migration in between the fetch of curcpu
and the call to spinlock_enter() by saving curcpu in a local variable.

- Use memory barriers to prevent reordering of loads and stores of the
data protected by the lock outside of the critical section

- Eliminate false sharing of the locks by moving them into the structures
that they protect and aligning them to a cacheline boundary.

- Record the owning thread in the lock to make debugging future problems
easier.

Reviewed by: rpaulo (initial version)
MFC after: 2 weeks

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Define dtrace_cmpset_long in terms of atomic_cmpset_long
and not by virtue of inline assembly. Now this file
compiles on all supported architectures.

Update several places that iterate over CPUs to use CPU_FOREACH().

Custom DTrace kernel module files plus FreeBSD-specific DTrace providers.