[SPARC]: Fix exec failures on sun4c.

This deals with a sun4c issue caused by commit b6a2fea3:
mm: variable length argument support.

The new way the code works means that sun4c_update_mmu_cache gets
called before a context has been selected, which results in invalid
operation of the underling mm code.

Simply ignoring update requests when there is no valid context solves
the problem.

Signed-off-by Mark Fortescue <mark@mtfhpc.demon.co.uk>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

arch/sparc/mm/sun4c.c

@@ -1999,6 +1999,9 @@ void sun4c_update_mmu_cache(struct vm_area_struct *vma, unsigned long address, p
 	unsigned long flags;
 	int pseg;
 
+	if (vma->vm_mm->context == NO_CONTEXT)
+		return;
+
 	local_irq_save(flags);
 	address &= PAGE_MASK;
 	if ((pseg = sun4c_get_segmap(address)) == invalid_segment) {