fix ip_input m_len assertion

[Sepherosa Ziehau]

Hi all,

Following patch fixes:
- The assertion in ip_input, should test sizeof(struct ip) instead of sizeof(ip)
- Make sure that ip_input's precondition meets in ip_localforward
- Don't allow raw ip socket to send ip packet whose header length is
less than the minimum

http://leaf.dragonflybsd.org/~sephe/ip_assert.diff

Please test/review.

Best Regards,
sephe

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