Improve default vnode operation handling for Coda:
Improve default vnode operation handling for Coda:

- Don't specify vnode operations for mknod, lease, and advlock--let them
  fall through to vop_default.

- Implement vop_default with &default_vnodeops, rather than with VOP_PANIC,
  so that unimplemented vnode operations are handled in more sensible ways
  than panicking, such as EOPNOTSUPP on ACL queries generated by bsdtar,
  or mknod.

MFC after:	3 days

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