SPARC64: Fix FPU saving in 64-bit signal handling.

Upstream commit: 7c3cce97

The calculation of the FPU reg save area pointer
was wrong.

Based upon an OOPS report from Tom Callaway.

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>