In linker_load_module(), check that rootdev != NODEV before calling linker_search_module().
In linker_load_module(), check that rootdev != NODEV before calling
linker_search_module().

Without this, modules loaded from loader.conf that then try to load
in additional modules (such as digi.ko loading a card's BIOS) die
badly in the vn_open() called from linker_search_module().

It may be worth checking (KASSERTing?) that rootdev != NODEV in
vn_open() too.

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