LU-622 Alloc enough cfg buffer space

LU-622 Alloc enough cfg buffer space

obd device name needs space enough for "<dev_name>-<instance_name>\0".
This fixes the glitch in commit cbc4ca2e8dc37d54bb7d3c9b02ab20b63e60f592.

Change-Id: Ib8c3c8db49baad8a9ca068a1c8009aee72aa1e62
Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/1280
Tested-by: Hudson
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Jinshan Xiong <jay@whamcloud.com>
Reviewed-by: Yu Jian <yujian@whamcloud.com>
Tested-by: Yu Jian <yujian@whamcloud.com>