e1000: Fix and clean up usage of DMA and TSO segments

[marius]

Summary

• At Intel it is believed that most of their products support "only" 40 DMA segments so lower {EM,IGB}_MAX_SCATTER accordingly. Actually, 40 is more than plenty to handle full size TSO packets so it doesn't make sense to further distinguish between MAC variants that really can do 64 DMA segments. Moreover, capping at 40 DMA segments limits the stack usage of {em,igb}_xmit() that - given the rare use of more than these - previously hardly was justifiable, while still being sufficient to avoid the problems seen with em(4) and EM_MAX_SCATTER set to 32.
• In igb(4), pass the actually supported TSO parameters up the stack. Previously, the defaults set in if_attach_internal() were applied, i. e. a maximum of 35 TSO segments, which made supporting more than these in the driver pointless. However, this might explain why no problems were seen with IGB_MAX_SCATTER at 64.
• In em(4), take the 5 m_pullup(9) invocations performed by em_xmit() in the TSO case into account when reporting TSO parameters upwards. In the worst case, each of these calls will add another mbuf and, thus, the requirement for an additional DMA segment. So for best performance, it doesn't make sense to advertize a maximum of TSO segments that typically will require defragmentation in em_xmit(). Again, this leaves enough room to handle full size TSO packets.
• Drop TSO macros from if_lem.h given that corresponding MACS don't support TSO in the first place.

[erj]

sys/dev/e1000/if_igb.c

3145 ↗

(On Diff #13164)

You don't want to set this value to IGB_MAX_SCATTER - 5, like in em?

[marius]

sys/dev/e1000/if_igb.c

3145 ↗

(On Diff #13164)

No, because unlike em_xmit(), igb_xmit() doesn't need to do m_pullup(9)'s adding to the count of segments required (the reverse, i. e. the em(4) case, is explained in the summary).
In principle, IGB_MAX_SCATTER could be lowered to 35, though, given there's no actual need for that extra 5 segments.

[jeffrey.e.pieper_intel.com]

I'm not seeing any issues with the testing I'm doing, so looks good to me.