if_wg: return to m_defrag() of incoming mbuf, sans leak

This partially reverts df55485085 but still fixes the leak. It was
overlooked (sigh) that some packets will exceed MHLEN and cannot be
physically contiguous without clustering, but we don't actually need
it to be. m_defrag() should pull up enough for any of the headers that
we do need to be accessible.

Fixes: df55485085
Pointy hat; kevans