do not call m_freem() inside mbq_lock()
do not call m_freem() inside mbq_lock()

On FreeBSD, at least, m_freem() eventually
calls a sleeping lock, while mbq_lock() uses
a spinlock.

luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap