ring-buffer: less locking and only disable preemption

Ingo,

These patches need to be put through the ringer. Could you add them
to your ring-buffer branch, so we can test them out before putting
them into your master branch.

The following patches bring the ring buffer closer to a lockless
solution. They move the locking only to the actual moving the
tail/write pointer from one page to the next. Interrupts are now
enabled during most of the writes.

A lot of the locking protection is still within the ftrace infrastructure.
The last patch takes some of that away.

The function tracer cannot be reentrant just due to the nature that
it traces everything, and can cause recursion issues.

-- Steve