When removing pages from the ring buffer, its state is not reset. This
means that the counters need to be correctly updated to account for the
pages removed.

Update the overrun counter to reflect the removed events from the pages.

Link: http://lkml.kernel.org/r/1340998301-1715-1-git-send-email-vnagarnaik@google.com

Cc: Justin Teravest <teravest@google.com>
Cc: David Sharp <dhsharp@google.com>
Signed-off-by: Vaibhav Nagarnaik <vnagarnaik@google.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>