The if_check() function performed three actions:

• verified that the ifp->if_snd.ifq_mtx was initalized for all attached interfaces. This was pointless because it was initalized for all interfaces in if_attach() so I've removed it.
• Checked that ifp->if_snd.ifq_maxlen is initalized and set it to ifqmaxlen if unset. This makes more sense in if_attach() so I moved it there.
• The first call of if_slowtimo(). Delete if_check() and call if_slowtimo() directly from the SYSINIT().