locking: Move mcs_spinlock.h into kernel/locking/

The mcs_spinlock code is not meant (or suitable) as a generic locking
primitive, therefore take it away from the normal includes and place
it in kernel/locking/.

This way the locking primitives implemented there can use it as part
of their implementation but we do not risk it getting used
inapropriately.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-byirmpamgr7h25m5kyavwpzx@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>