[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: David S. Miller <davem@davemloft.net>