LU-15727 lod: honor append_pool with default composite layouts

LU-15727 lod: honor append_pool with default composite layouts

In lod_get_default_lov_striping(), correct the handling of composite
default layouts in the case where append_stripe_count is nonzero.
Align the names of the append members of struct dt_allocation_hint
with the mdd params. Remove the unused dah_mode member of struct
dt_allocation_hint.

Add sanity test_27U() to verify.

Fixes: e2ac6e1eaa ("LU-9341 lod: Add special O_APPEND striping")

Lustre-change: https://review.whamcloud.com/47014
Lustre-commit: 03963106926883cf322e085feb8caa3ea64db1d1

Change-Id: I66b426d24d6476fb483397f290229983f3da4be5
Signed-off-by: Xing Huang <hxing@ddn.com>
Reviewed-on: https://review.whamcloud.com/48027
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>