tracing: Remove unnecessary WARN_ONCE's from tracing_buffers_splice_read

WARN shouldn't be used as a means of communicating failure to a userspace programmer.

Link: http://lkml.kernel.org/r/20120725153908.GA25203@redhat.com



Signed-off-by: Dave Jones <davej@redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>