net/mlx5e: XDP, Close TX MPWQE session when no room for inline packet left

In MPWQE mode, when transmitting packets with XDP, a packet that is smaller
than a certain size (set to 256 bytes) would be sent inline within its WQE
TX descriptor (mem-copied), in case the hardware tx queue is congested
beyond a pre-defined water-mark.

If a MPWQE cannot contain an additional inline packet, we close this
MPWQE session, and send the packet inlined within the next MPWQE.
To save some MPWQE session close+open operations, we don't open MPWQE
sessions that are contiguously smaller than certain size (set to the
HW MPWQE maximum size). If there isn't enough contiguous room in the
send queue, we fill it with NOPs and wrap the send queue index around.

This way, qualified packets are always sent inline.

Perf tests:
Tested packet rate for UDP 64Byte multi-stream
over two dual port ConnectX-5 100Gbps NICs.
CPU: Intel(R) Xeon(R) CPU E5-2680 v3 @ 2.50GHz

XDP_TX:

With 24 channels:
| ------ | bounced packets | inlined packets | inline ratio |
| before | 113.6Mpps       | 96.3Mpps        | 84%          |
| after  |   115Mpps       | 99.5Mpps        | 86%          |

With one channel:

| ------ | bounced packets | inlined packets | inline ratio |
| before | 6.7Mpps         | 0pps            | 0%           |
| after  | 6.8Mpps         | 0pps            | 0%           |

As we can see, there is improvement in both inline ratio and overall
packet rate for 24 channels. Also, we see no degradation for the
one-channel case.

Signed-off-by: Shay Agroskin <shayag@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>