ip_output(): style


ip_output(): if mb_unmapped_to_ext() failed, return directly

do not free the original mbuf, it is already freed by the
mb_unmapped_to_ext().


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().