Fix special case in ip_fragment() to produce a more sensible chain of packets

[• hselasky]

Summary

When the MTU on a network adapter is greater than PAGE_SIZE a special case in ip_fragment() can kick in to optimise the payloads.

Setup:
MTU = 5K
PAGE_SIZE = 4K

Send a ping packet of 7K.
The receive of the network adapter can use multiple MBUFs to receive a 7K payload, for example:
2k + 4k + 1k
When the packet gets echoed back, we end up in ip_fragment() which the split the packets oddly, because it assumes that each MBUF in the m_next chain ends on an MTU, which is not a requirement?!
This patch fixes the computation, so that the resulting transmitted packet chain will look like:
4k + 3k

[• hselasky]

Add full context.

[• hselasky]

Use MIN() macro.

[glebius]

This is Ken's code, we probably want him to look at. Looks like a remnant from zero copy era.

[ken]

It looks okay to me. The surrounding code has changed a good bit since that was committed in 2002. By the way, I think Drew Gallatin originally wrote that chunk of code, but I can't remember for sure.

[• hselasky]

Committed to FreeBSD-head in r279281.