sparc64: Fix stack tracing through trap frames.

The offset to the pt_regs area was wrong, so we weren't
looking at the right location for the magic cookie.

A trap frame is composed of a "struct sparc_stackf" then
a "struct pt_regs", the code was using "struct reg_window"
instead of "struct sparc_stackf".

Signed-off-by: David S. Miller <davem@davemloft.net>