mb_unmapped_to_ext(): fix m_free()/m_freem() use

The top of the chain is supposed to be freed in mb_unmapped_to_ext(),
use m_freem(). The _mb_unmapped_to_ext() helper only needs to work on
the passed mbuf, use m_free().

Reviewed by: glebius
Sponsored by: NVidia networking
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D49305