Previously, kdump used the kernel-only timervalsub() macro which required
defining _KERNEL when including <sys/time.h>. Now, kdump uses the existing
userland API. The timercmp() usage to check for a backwards timestamp is
also clearer and simpler than the previous code which checked the result of
the subtraction for a negative value.
While here, take advantage of the 3-arg timersub() to store the subtraction
results in a tempory timeval instead of overwriting the timestamp in the
ktrace record and then having to restore it.
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