futex: unlock before returning -EFAULT

Impact: rt-mutex failure case fix

futex_lock_pi can potentially return -EFAULT with the rt_mutex
held.  This seems like the wrong thing to do as userspace should
assume -EFAULT means the lock was not taken.  Even if it could
figure this out, we'd be leaving the pi_state->owner in an
inconsistent state.  This patch unlocks the rt_mutex prior to
returning -EFAULT to userspace.

Build and boot tested on a 4 way Intel x86_64 workstation.
Passes basic pthread_mutex and PI tests out of
ltp/testcases/realtime.

Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
LKML-Reference: <20090312075606.9856.88729.stgit@Aeon>
Signed-off-by: Ingo Molnar <mingo@elte.hu>