drivers/ptp: Fix kernel memory disclosure

The reserved field precise_offset->rsv is not cleared before being
copied to user space, leaking kernel stack memory. Clear the struct
before it's copied.

Signed-off-by: Vlad Tsyrklevich <vlad@tsyrklevich.net>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>