There is and has been for a very long time been a lot more going on in
flush_old_exec than just flushing the old state.  After the movement
of code from setup_new_exec there is a whole lot more going on than
just flushing the old executables state.

Rename flush_old_exec to begin_new_exec to more accurately reflect
what this function does.
Reviewed-by: NKees Cook <keescook@chromium.org>
Reviewed-by: NGreg Ungerer <gerg@linux-m68k.org>
Signed-off-by: N"Eric W. Biederman" <ebiederm@xmission.com>