lib ring buffer: use irq_work for wakeup by writer

Using irq_work (like perf does) allows using an interrupt handler
firing soon after the instrumentation execution to issue the wakeups.

This allows the RING_BUFFER_WAKEUP_BY_WRITER ring buffer configuration
to be entirely lock-free, which allows using it in NMI context for
general tracing purposes.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I842ff15736f53d1283cf953804d803f70779652b