leaked memory on each unload and were limited to items referenced in
the kernel copy of vnode_if.c. Now a kernel module is free to create
it's own VOP_FOO() routines and the rest of the system will happily
deal with it, including passthrough layers like union/umap/etc.
Have VFS_SET() call a common vfs_modevent() handler rather than
inline duplicating the common code all over the place.
Have VNODEOP_SET() have the vnodeops removed at unload time (assuming a
module) so that the vop_t ** vector is reclaimed.
Slightly adjust the vop_t ** vectors so that calling slot 0 is a panic
rather than a page fault. This could happen if VOP_something() was called
without *any* handlers being present anywhere (including in vfs_default.c).
slot 1 becomes the default vector for the vnodeop table.
TODO: reclaim zones on unload (eg: nfs code)
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