When creating macvtaps that are expected to have the same ifindex
in different network namespaces, only the first one will succeed.
The others will fail with a sysfs_warn_dup warning due to them trying
to create the following sysfs link (with 'NN' the ifindex of macvtapX):

/sys/class/macvtap/tapNN -> /sys/devices/virtual/net/macvtapX/tapNN

This is reproducible by running the following commands:

ip netns add ns1
ip netns add ns2
ip link add veth0 type veth peer name veth1
ip link set veth0 netns ns1
ip link set veth1 netns ns2
ip netns exec ns1 ip l add link veth0 macvtap0 type macvtap
ip netns exec ns2 ip l add link veth1 macvtap1 type macvtap

The last command will fail with "RTNETLINK answers: File exists" (along
with the kernel warning) but retrying it will work because the ifindex
was incremented.

The 'net' device class is isolated between network namespaces so each
one has its own hierarchy of net devices.
This isn't the case for the 'macvtap' device class.
The problem occurs half-way through the netdev registration, when
`macvtap_device_event` is called-back to create the 'tapNN' macvtap
class device under the 'macvtapX' net class device.

This patch adds namespace support the the 'macvtap' device class so
that /sys/class/macvtap is no longer shared between net namespaces.

However, doing this has the side effect of changing
/sys/devices/virtual/net/macvtapX/tapNN  into
/sys/devices/virtual/net/macvtapX/macvtap/tapNN

This is due to Commit 24b1442 ("Driver-core: Always create class
directories for classses that support namespaces.")