LU-11757 lod: use calculated stripe count

LU-11757 lod: use calculated stripe count

lod_prep_md_striped_create() tries to allocat big
chunk of memory because
lum->lum_stripe_count == -1 and is converted to __u32.

ldo_dir_stripe_count was calculated already in lod_ah_init()

Lustre-change: https://review.whamcloud.com/33829
Lustre-commit: 622a94d5e27ed3e596918863c08b304a6be9a646

Change-Id: Id99d9e024638dfb1b34262840d2e543c808a9cdc
Cray-bug-id: LUS-6694
Signed-off-by: Andriy Skulysh <c17819@cray.com>
Reviewed-by: Alexander Boyko <c17825@cray.com>
Tested-by: Elena Gryaznova <c17455@cray.com>
Reviewed-by: Vladimir Saveliev <c17830@cray.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/33981
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>