sysctl extern cleanup: rtmutex

Extern declarations in sysctl.c should be moved to their own header file,
and then include them in relavant .c files.

Move max_lock_depth extern declaration to linux/rtmutex.h

Signed-off-by: Dave Young <hidave.darkstar@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>