futex: make futex_(get|put)_key() calls symmetric

Impact: cleanup

This patch makes the calls to futex_get_key_refs() and futex_drop_key_refs()
explicitly symmetric by only "putting" keys we successfully "got".  Also
cleanup a couple return points that didn't "put" after a successful "get".

Build and boot tested on an x86_64 system.

Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
Cc: <stable@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>