sparc/ptrace: Preserve previous registers for short regset write

commit d3805c546b275c8cc7d40f759d029ae92c7175f2 upstream.

Ensure that if userspace supplies insufficient data to PTRACE_SETREGSET
to fill all the registers, the thread's old registers are preserved.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>