drivers/vfio: Rework offsetofend()

commit b13460b92093b29347e99d6c3242e350052b62cd upstream.

The macro offsetofend() introduces unnecessary temporary variable
"tmp". The patch avoids that and saves a bit memory in stack.

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
[wt: backported only for ipv6 out-of-bounds fix]

Signed-off-by: Willy Tarreau <w@1wt.eu>