net: if: Increment the packet ref count when adding to timestamp queue

Added incrementation of the packet reference count when puting
the packet on the queue used in tx timestamping thread. This fixes
an issue when user wants to access the packet data in the timestamp
callback context. Before the fix was introduced if sockets were used
packet has been unreferenced before execution reached timestamp callback
context.

Signed-off-by: Adam Wojasinski <awojasinski@baylibre.com>