LU-744 llite: reimplement ll_get_fsname()

LU-744 llite: reimplement ll_get_fsname()

ll_get_fsname() used to allocate a piece of memory to store fsname,
this is not needed and error prone because it requires the caller
to free that piece of memory.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: Ibc077b46728a1358e51a345ec13c966fc947c428
Reviewed-on: http://review.whamcloud.com/3704
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>