MN10300: Fix ret_from_kernel_thread

ret_from_kernel_thread needs to set A2 to the thread_info pointer before
jumping to syscall_exit.

Without this, we never correctly start userspace.

This was caused by the rejuggling of the fork/exec paths in commit
ddf23e87 ("mn10300: switch to saner kernel_execve() semantics")

Reported-by: Ken Cox <jkc@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Ken Cox <jkc@redhat.com>
Acked-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>