using VOP_LOOKUP() when VFS_VGET() returns EOPNOTSUPP in the
ReaddirPlus RPC. This patch is based upon one by pjd@ for the
regular nfs server which has not yet been committed. It is needed
when a ZFS volume is exported and ReaddirPlus (which almost
always happens for NFSv4) is performed by a client. The patch
also simplifies vnode lock handling somewhat.
MFC after: 2 weeks