Staging: lustre: lov: check return value of lov_sub_get()

Check return value of lov_sub_get() in lov_io_read_ahead().

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>