make sure that we always have a return path from kernel_execve()

The only place where kernel_execve() is called without a way to
return to the caller of kernel_thread() callback is kernel_post().
Reorganize kernel_init()/kernel_post() - instead of the former
calling the latter in the end (and getting freed by it), have the
latter *begin* with calling the former (and turn the latter into
kernel_thread() callback, of course).

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>