Revert m_defrag() mbuf alignment to IP header.

Revert m_defrag() mbuf alignment to IP header.

m_defrag() is intended as last resort to make DMA transfers to the
hardware.  Therefore page alingment is more important than IP header
alignment.  The reason, why the mbuf returned by m_defrag() was
switched to IP header alingment, was that ether_extract_headers()
failed in em(4) driver with TSO on sparc64.  This has been fixed
by using memcpy().

The alignment change in m_defrag() is too late in the 7.5 relaese
process.  It may affect several drivers on different architectures.
Bus dmamap for ixl(4) on sun4v expects page alignment.  Such alignment
issues and TSO mbuf mapping for IOMMU need more thought.

OK deraadt@