freebsd: worker_lock: use MTX_DEF type in place of MTX_SPIN
freebsd: worker_lock: use MTX_DEF type in place of MTX_SPIN

This is safe because lock is taken by the worker thread or by the
interrupt threads calling netmap_[tr]x_irq(), where it is safe
to sleep.

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