net/core: Add drop counters to VF statistics

Modern hardware can decide to drop packets going to/from a VF.
Add receive and transmit drop counters to be displayed at hypervisor
layer in iproute2 per VF statistics.

Signed-off-by: Eugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>