locking/doc: Update references to kernel/mutex.c

Fix this docbook error:

  >> docproc: kernel/mutex.c: No such file or directory

by updating the stale references to kernel/mutex.c.

Reported-by: fengguang.wu@intel.com
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-34pikw1tlsskj65rrt5iusrq@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --cc Documentation/DocBook/kernel-locking.tmpl
Simple merge

diff --cc Documentation/mutex-design.txt
Simple merge

diff --cc include/linux/mutex.h
Simple merge

diff --cc kernel/locking/mutex.c
Simple merge