sparc64: Properly truncate pt_regs framepointer in perf callback.

For 32-bit processes, we save the full 64-bits of the regs in pt_regs.

But unlike when the userspace actually does load and store
instructions, the top 32-bits don't get automatically truncated by the
cpu in kernel mode (because the kernel doesn't execute with PSTATE_AM
address masking enabled).

So we have to do it by hand.

Reported-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>