Fix extent rounding for striped segments.

Fix extent rounding for striped segments.

We should never remove more extents than requested by user,
so round up to next stripe boundary during lvreduce.

Also this fixes round to zero sized LV bug:

# lvcreate -i2 -I 64k -l10 -n lvs vg_test
# lvreduce -f -l1 vg_test/lvs
  Rounding size (1 extents) down to stripe boundary size for segment (0 extents)
  WARNING: Reducing active logical volume to 0
  THIS MAY DESTROY YOUR DATA (filesystem etc.)
  Reducing logical volume lvs to 0
  Failed to suspend lvs