nullfs: do not allow bypass on copy_file_range()

There must be no callers of VOP_COPY_FILE_RANGE() except
vn_copy_file_range(), which does enough to find the write-vnodes where
to call the VOP.

Reviewed by:	markj, Olivier Certner <olce.freebsd@certner.fr>
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D42603

sys/fs/nullfs/null_vnops.c

@@ -1177,5 +1177,6 @@ struct vop_vector null_vnodeops = {
 	.vop_vptofh =		null_vptofh,
 	.vop_add_writecount =	null_add_writecount,
 	.vop_vput_pair =	null_vput_pair,
+	.vop_copy_file_range =	VOP_PANIC,
 };
 VFS_VOP_VECTOR_REGISTER(null_vnodeops);