Introduce lo_mtx to protect the global loopback softc list.  I'm not really sure why we have a softc list for if_loop, given that it can't be unloaded, but that's an issue to revisit in the future as corrupting the softc list would still cause panics.
Introduce lo_mtx to protect the global loopback softc list.  I'm not
really sure why we have a softc list for if_loop, given that it
can't be unloaded, but that's an issue to revisit in the future as
corrupting the softc list would still cause panics.

Reviewed by:	benno

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