futex: clean up futex_(un)lock_pi fault handling

Impact: cleanup

Some apparently left over cruft code was complicating the fault logic:

Testing if uval != -EFAULT doesn't have any meaning, get_user() sets ret
to either 0 or -EFAULT, there's no need to compare uval, especially not
against EFAULT which it will never be.  This patch removes the superfluous
test and clarifies the comment blocks.

Build and boot tested on an 8way x86_64 system.

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