locking/semaphore: Update the file path in documentation

While reading this header I noticed that the locking stuff has moved to
kernel/locking/*, so update the path in semaphore.h to point to that.

Signed-off-by: Tycho Andersen <tycho@tycho.ws>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20180201114119.1090-1-tycho@tycho.ws


Signed-off-by: Ingo Molnar <mingo@kernel.org>