mbuf: provide mc_uiotomc() a function to copy from uio(9) to mchain

Implement m_uiotombuf() as a wrapper around mc_uiotomc(). The M_EXTPG is
left untouched. The m_uiotombuf() is left as a compat KPI. New code
should use either mc_uiotomc() or m_uiotombuf_nomap().

Reviewed by: markj, tuexen
Differential Revision: https://reviews.freebsd.org/D44150