Keep mbuf data alignment intact in m_defrag()

Keep mbuf data alignment intact in m_defrag()

The recent TSO support in em(4) triggered an alignment error on the TCP
header. In em(4) m_defrag() is called before setting up the TSO dma bits
and with that the TCP header was suddenly no longer aligned. Like other
mbuf functions preserve the data alignment in m_defrag() to prevent such
unaligned packets.

With help and OK bluhm@ mglocker@