s390: fold kernel_thread_helper() into ret_from_fork()

... and don't bother with syscall return path in case of kernel
threads.

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

diff --cc arch/s390/kernel/entry.S
Simple merge

diff --cc arch/s390/kernel/entry64.S
Simple merge

diff --cc arch/s390/kernel/process.c
Simple merge