ptp: check that rsv field is zero in struct ptp_sys_offset_extended

Otherwise it is impossible to use it for something else, as it will break
userspace that puts garbage there.

The same check should be done in other structures, but the fact that
data in reserved fields is ignored is already part of the kernel ABI.

Signed-off-by: Eugene Syromiatnikov <esyr@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>