Handler threads use a selection of mirror functions with the
assumption that the data referred to is RCU protected, while the
implementation has not provided for this, which can lead to an OVS
crash.

This patch fixes this by making the mbundle lookup RCU-safe by using
cmap instead of hmap and postponing mbundle memory free, as wells as
postponing the frees of the mirrors and the vlan bitmaps of each
mirror.

Note that mirror stats update is still not accurate if multiple
threads do it simultaneously.

A less complete version of this patch (using cmap and RCU postpone
just for the mbridge itself) was tested by Yunjian Wang and was found
to fix the observed crash when running a script that adds and deletes
a port repeatedly.

Reported-by: Yunjian Wang <wangyunjian@huawei.com>
Signed-off-by: Jarno Rajahalme <jarno@ovn.org>