vtnet: Adjust rx buffer so IP header 32-bit aligned

Call madj(m, ETHER_ALIGN) to offset rx buffers when allocating them.
This improves performance everywhere, and allows armv7 to work at all.

PR: 271288 (PR had a different fix than I wound up with)
MFC After: 3 days
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D43136