Change a very strange code in m_demote() to simple assertion.

Sponsored by:	Nginx, Inc.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271122 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Satisfy assertion in m_demote().

Sponsored by:	Nginx, Inc.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271123 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f