exec: Rename the flag called_exec_mmap point_of_no_return

Update the comments and make the code easier to understand by
renaming this flag.

Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Greg Ungerer <gerg@linux-m68k.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>