rpc_new_dir(): the last argument is always NULL

rpc_new_dir(): the last argument is always NULL

All callers except the one in rpc_populate() pass explicit NULL
there; rpc_populate() passes its last argument ('private') instead,
but in the only call of rpc_populate() that creates any subdirectories
(when creating fixed subdirectories of root) private itself is NULL.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>