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
ddf23e87a804 ("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>

arch/mn10300/kernel/entry.S

@@ -60,6 +60,7 @@ ENTRY(ret_from_kernel_thread)
 	mov	(REG_D0,fp),d0
 	mov	(REG_A0,fp),a0
 	calls	(a0)
+	GET_THREAD_INFO a2		# A2 must be set on return from sys_exit()
 	clr	d0
 	mov	d0,(REG_D0,fp)
 	jmp	syscall_exit