BUG-974 handle overflow in packet sending for speed_packetrate and speed_mbpsrate

[plangarb]

Standards checklist:

• The PR title is descriptive.
• The PR doesn't replicate another PR which is already open.
• The code is mine or it's from somewhere with an MIT-compatible license.
• The code is efficient, to the best of my ability, and does not waste computer resources.
• The code is stable and I have tested it myself, to the best of my abilities.
• The code passes sudo make test

Changes:

• Convert to nanosec after division

Other comments:

Test result: ~8GB pcap file, tcpreplay command using speed_mbpsrate = 10. Speed rate is constant throughout the entire operation.

tcpreplay -i veth0 -M 10 --stats=1 8GB_test_file.pcap

Logs

Actual: 1634674 packets (8578746889 bytes) sent in 6862.99 seconds
Rated: 1250000.0 Bps, 10.00 Mbps, 238.18 pps
Actual: 1634916 packets (8579997033 bytes) sent in 6863.99 seconds
Rated: 1250000.1 Bps, 10.00 Mbps, 238.18 pps
Actual: 1635160 packets (8581249984 bytes) sent in 6865.00 seconds
Rated: 1249999.9 Bps, 9.99 Mbps, 238.18 pps
Actual: 1635403 packets (8582503398 bytes) sent in 6866.00 seconds
Rated: 1250000.1 Bps, 10.00 Mbps, 238.18 pps
Actual: 1635646 packets (8583758340 bytes) sent in 6867.00 seconds
Rated: 1250000.1 Bps, 10.00 Mbps, 238.18 pps
Actual: 1635883 packets (8585008943 bytes) sent in 6868.00 seconds
Rated: 1250000.0 Bps, 10.00 Mbps, 238.18 pps
Actual: 1636123 packets (8586262031 bytes) sent in 6869.00 seconds
Rated: 1250000.1 Bps, 10.00 Mbps, 238.18 pps
Actual: 1636353 packets (8587513641 bytes) sent in 6870.01 seconds
Rated: 1249999.9 Bps, 9.99 Mbps, 238.18 pps
Actual: 1636592 packets (8588765064 bytes) sent in 6871.01 seconds
Rated: 1250000.0 Bps, 10.00 Mbps, 238.18 pps
Test complete: 1970-01-01 02:02:32.532258497
Actual: 1636812 packets (8589935953 bytes) sent in 6871.94 seconds
Rated: 1249999.9 Bps, 9.99 Mbps, 238.18 pps
Flows: 1 flows, 0.00 fps, 1636812 unique flow packets, 0 unique non-flow packets
Statistics for network device: veth0
Successful packets: 1636812
Failed packets: 0
Truncated packets: 0
Retried packets (ENOBUFS): 0