LU-12852 pfl: restrict the stripe count correctly

In function lod_get_stripe_count(), when restricting the stripe
count to the maximum xattr size, the xattr overhead should be
taken into count correctly.

Signed-off-by: Emoly Liu <emoly@whamcloud.com>
Change-Id: Ief548e47ce4d375f2e189860ccfe05d0f3c7e890
Reviewed-on: https://review.whamcloud.com/36947
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>