[PATCH] initialise pi_lock if CONFIG_RT_MUTEXES=N

Fixes a bogus lockdep warning which causes lockdep to disable itself.

Acked-by: Ingo Molnar <mingo@elte.hu>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>