futex: fix futex_wait_setup key handling

If the get_futex_key() call were to fail, the existing code would
try and put_futex_key() prior to returning.  This patch makes sure
we only put_futex_key() if get_futex_key() succeeded.

Reported-by: Clark Williams <williams@redhat.com>
Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
LKML-Reference: <20090410165005.14342.16973.stgit@Aeon>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>