Add support for sysctl knobs to live tune the tx packet processing limits in igb and fix a wrap-around bug.

[sbruno]

Summary

Prior this was using limit which was simply delcared but not initialized. With it being a u16, it would wrap around to 65535 after the first loop,
meaning the old limit was really 65536 packets and not truly unlimited.
With a 1G card, it shouldn't make much a difference, but initialize it
to -1 to be safe and more clear on the setting which matches the tool tip.

Add tx processing limit sysctl to igb(4)

[hiren]

Is there a way we can unify *_process_limit and *_processing_limit? I see both being used and that is confusing. I am not sure if its just the code and if user is presented with only one but in any case we should choose one and stick with it. BTW, this can be done as a followup/separate commit.

This change looks okay to me.

[j-nitrology.com]

In D4039#84487, @hiren wrote:

Is there a way we can unify *_process_limit and *_processing_limit? I see both being used and that is confusing. I am not sure if its just the code and if user is presented with only one but in any case we should choose one and stick with it. BTW, this can be done as a followup/separate commit.

This change looks okay to me.

So the process_limit is the global boot time tunable that becomes the default per adapter setting that can (now) be tuned live. That per adapter setting is called processing_limit. I agree though, we can probably give them some better naming. For it to make sense we'd probably want to change the sysctls to match, which I don't believe is safe:

hw.igb.tx_process_limit: -1
hw.igb.rx_process_limit: -1

dev.igb.1.tx_processing_limit: -1
dev.igb.1.rx_processing_limit: -1
dev.igb.0.tx_processing_limit: -1
dev.igb.0.rx_processing_limit: -1