Abstract out the location of an m_tag's data by adding a m_tag_data() inline.

Abstract out the location of an m_tag's data by adding a m_tag_data() inline.
Replace all instances of 'mtag + 1' with the new inline.

Fix numerous bugs, mainly in ipfw/ipfw2, where the m_tag data was being stored
in the wrong place and corrupting the m_tag, resulting in a panic.  This
primarily occured with the use of divert rules.

Reported-by: Ben Woolley <tautolog@gmail.com>