net/mlx5: add representor recognition on Linux 5.x

net/mlx5: add representor recognition on Linux 5.x

The master device and VF representors were distinguished by
presence of port name, master device did not have one. The new Linux
kernels starting from 5.0 provide the port name for master device
and the implemented representor recognizing method does not work.
The new recognizing method is based on querying the VF number,
has been created on the base of the device.

The IFLA_NUM_VF attribute is returned by kernel if IFLA_EXT_MASK
attribute is specified in the Netlink request message.

Also the presence check of device symlink in device sysfs folder
is added to distinguish representors with sysfs based method.

Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Shahaf Shuler <shahafs@mellanox.com>