m68k: switch to generic sys_execve()/kernel_execve()

The tricky part here is that task_pt_regs() on m68k works *only* for
process inside do_signal().  However, we need something much simpler -
pt_regs of a process inside do_signal() may be at different offsets
from the stack bottom, depending on the way we'd entered the kernel,
but for a task inside sys_execve() it *is* at constant offset.
Moreover, for a kernel thread about to become a userland process the
same location is also fine - setting sp to that will leave the kernel
stack pointer at the very bottom of the kernel stack when we finally
switch to userland.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/arch/m68k/kernel/entry.S b/arch/m68k/kernel/entry.S
index 8a01f58..946cb01 100644
--- a/arch/m68k/kernel/entry.S
+++ b/arch/m68k/kernel/entry.S
@@ -122,6 +122,11 @@
 	movel	%d0,(%sp)
 	jra	sys_exit
 
+ENTRY(ret_from_kernel_execve)
+	movel	4(%sp), %sp
+	GET_CURRENT(%d0)
+	jra	ret_from_exception
+
 #if defined(CONFIG_COLDFIRE) || !defined(CONFIG_MMU)
 
 #ifdef TRAP_DBG_INTERRUPT