openvswitch: Use nla_memcpy() to memcpy() data from attributes

Less error prone as it takes into account the length of both the
destination buffer and the source attribute and documents when
data is copied from an attribute.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Jesse Gross <jesse@nicira.com>

diff --cc net/openvswitch/datapath.c
Simple merge

diff --cc net/openvswitch/flow.c
Simple merge