locking: Move the rtmutex code to kernel/locking/

Suggested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-p9ijt8div0hwldexwfm4nlhj@git.kernel.org


[ Fixed build failure in kernel/rcu/tree_plugin.h. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>