LU-17713 mdd: validate the length of mdd append_pool name

Validate the length of mdd append_pool name (<= LOV_MAXPOOLNAME)
before saving it in function append_pool_store().
Also, sanity.sh test_27M is improved a little to verify this fix.

Signed-off-by: Emoly Liu <emoly@whamcloud.com>
Change-Id: Id7083fab60e9a18af4d8eedfa3d55f37544ba15d
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54691
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Zhenyu Xu <bobijam@hotmail.com>