Merge proc_cmdline simplifications.

This re-writes the get_mm_cmdline() logic to be rather simpler than it
used to be, and makes the semantics for "cmdline goes past the end of
the original area" more natural.

You _can_ use prctl(PR_SET_MM) to just point your command line somewhere
else entirely, but the traditional model is to just edit things in place
and that still needs to continue to work.  At least this way the code
makes some sense.

* proc-cmdline:
  fs/proc: simplify and clarify get_mm_cmdline() function
  fs/proc: re-factor proc_pid_cmdline_read() a bit