LU-12043 llite: switch to use ll_fsname directly

LU-12043 llite: switch to use ll_fsname directly

There are many places which try to access filesystem
fsname, instead of parsing it everytime, just store
it into @sbi, we could use @ll_fsname directly whenever we need.

Change-Id: I25b8505b9aac5d4f6e97fe3691321be2b800f35d
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/34602
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Jenkins
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>