Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r244631
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 r226452 (by marcel@)

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.

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

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

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