Use 3.17 ktime_get_mono_fast_ns() new API

The new ktime_get_mono_fast_ns() API introduced in kernel 3.17 allows
tracing NMI handlers.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cleanup: remove unused trace_clock_read32()

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>