tile/ptrace: Preserve previous registers for short regset write

commit fd7c99142d77dc4a851879a66715abf12a3193fb 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>
Signed-off-by: Chris Metcalf <cmetcalf@mellanox.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>