futex: Minor fixes to doc and code

Hello Thomas,

While working on revisions to the futex(2) man page, I noticed a couple of source files that could be improved. In one Doc file, important constants are misnamed, so that grepping won't find useful information. And in the futex.c code, the argument handling in futex_lock_pi() is a little confused.

Thanks,

Michael

Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/