[hns-1126]net: hns3: add check for max TX BD num for tso and non-tso case

[Fred Kimmy]

"[Bug Description]
Hardware supports up to 8 TX BD for non-TSO skb and 63 TX
BD for TSO skb. Currently hns3 driver does not check the max
BD num that required by a skb before filling desc, which may
cause the hardware to issue a RAS error throug PCIe AER.

[Steps to Reproduce]
1.load PF driver
2.send 655356 Bytes' non-TSO packets

[Actual Results]
The hardware issue a RAS error.

[Expected Results]
The driver drop packets.

[Reproducibility]
Inevitably

[Additional information]
Hardware: D06
Firmware: NA
Kernel: NA

[Resolution]
This patch adds the max BD num check before filling desc,
if the bd num is not within the hardware limit, it will
record the error by ring->stats.sw_err_cnt counter and
free the skb.
This patch also cleans up the hns3_nic_bd_num function by
changing the return type and removing an unnecessary check."

net: hns3: add check for max TX BD num for tso and non-tso case

[Ike Panhc (ikepanhc)]

I will try to reproduce this and see if we shall backport it.

[Ike Panhc (ikepanhc)]

`git cherry-pick` on bionic/disco/eoan fails with lots of conflict. Set back to "Triage" until we can solve the conflicts.

[Ike Panhc (ikepanhc)]

19.04 is EOLed. There is no more SRU cycle for it.

[Ike Panhc (ikepanhc)]

Can't clean cherry-picked for eoan and bionic.