breaking the global file_list_lock

This patch-set breaks up the global file_list_lock which was found to be a
severe contention point under basically any filesystem intensive workload.

It has been part of the -rt kernel for some time now and is deemed solid and
useful enough to post in its own right. This contention should also occur on
the large SMP machines.

Feedback would be appreciated, especially with regard to the extra workqueue
that was added to flush per cpu lists. Is there an alternative aproach?