tracing: Erase irqsoff trace with empty write

tracing: Erase irqsoff trace with empty write

One convenient way to erase trace is "echo > trace". However, this
is currently broken if the current tracer is irqsoff tracer. This
is because irqsoff tracer use max_buffer as the default trace
buffer.

Set the max_buffer as the one to be cleared when it's the trace
buffer currently in use.

Link: http://lkml.kernel.org/r/[email protected]
Cc: <[email protected]>
Cc: [email protected]
Fixes: 4acd4d00f ("tracing: give easy way to clear trace buffer")
Signed-off-by: Bo Yan <[email protected]>
Signed-off-by: Steven Rostedt (VMware) <[email protected]>