net/hns3: get number of used descriptors of Rx queue

net/hns3: get number of used descriptors of Rx queue

Implement the available and used rxd number count function.

In Kunpeng series, the NIC hardware supports to read the bd numbers
which wait processed from the hardware FBD (Full Buffer Descriptor),
and the driver maintains the bd number to be written back hardware.
Compare the number of FBDs with the number of BDs to be written back to
the hardware.

The number of used descriptors of a rx queue is computed as follows:
The fbd numbers of reading from FBD register plus the bd numbers to be
written back to hardware maintained by the driver.

Signed-off-by: Lijun Ou <oulijun@huawei.com>