perf tools: Fix ommitted mmap data update on remap

Commit eac9eace "perf tools: Check we are able to read the event
size on mmap" brought a check to ensure we can read the size of the
event before dereferencing it, and do a remap otherwise to move the
buffer forward.

However that remap was ommitting all the necessary work to
update the new page offset, head, and to unmap previous pages,
etc...

To fix this, gather all the code that fetches the event in a
seperate helper which does all the necessary checks about the
header/event size and tells us anytime a remap is needed.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1306148788-6179-3-git-send-email-fweisbec@gmail.com


Signed-off-by: Ingo Molnar <mingo@elte.hu>