sparc64: Handle 32-bit tasks properly in compute_effective_address().

[ Upstream commit d037d16372bbe4d580342bebbb8826821ad9edf0 ]

If we have a 32-bit task we must chop off the top 32-bits of the
64-bit value just as the cpu would.

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>