MFC r324673:
mbuf(9): unbreak m_fragment()

• Fix it by replacing m_cat() with m_prev->m_next = m_new

(m_cat() will try to append data - as a result, there will be no
fragmentation).

• Move some checks out of the loop.

Some variables were renamed (m_final -> m_first, m_new -> m_last)

Was previously tested with D4077.

Differential Revision: D4090