vectors; defer that to vfs_opv_init().
Change the interface to vfs_opv_init() and export it; it now takes a
pointer to an array of vnodeopv_desc *'s to initialize. Allocate
the vnode operations vectors here. Called by vfs_attach().
Implement vfs_opv_free(), which deallocates the vnode operations
vectors. Called by vfs_detach().
Change vfsinit() to build the initial vfs_list by traversing the
vfs_list_inital[] table, and vfs_attach()'ing those file systems.
Also, initialize special vnodeopv_descs (dead, fifo, spec) which
are not associated with any particular file system.
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