ZFS: use -p flag and remove zfs_parse_size

ZFS: use -p flag and remove zfs_parse_size

ZFS supports the -p flag in the list command since a few years now.
Let us use the real byte values and avoid the error prone calculation
from human readable numbers that can lead to incorrect numbers if the
reported human readable value is a rounded number.

Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>