net/af_xdp: fix umem size

net/af_xdp: fix umem size

The kernel expects the start address of the UMEM to be page size
aligned.
Since the mempool is not guaranteed to have such alignment, we have been
aligning the address to the start of the page the mempool is on. However
when passing the 'size' of the UMEM during it's creation we did not take
this into account.

This commit adds the amount by which the address was aligned to the size
of the UMEM.

Bugzilla ID: 532
Fixes: d8a210774e1d ("net/af_xdp: support unaligned umem chunks")
Cc: stable@dpdk.org
Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>