tasks/cephfs/mount: use 192.168.144.0.0/20 for brxnet

tasks/cephfs/mount: use 192.168.144.0.0/20 for brxnet

Most of wifi routers are using 192.168.0.0/16 network by default,
as well as in many lab setup this is most popular network range..
If someone tries and establish libvirt based teuthology setup
at home pc or laptop, for example, using downburst, they may
experience unnecessary troubles with executing tests which use
ceph-fuse tasks and cephfs/mount code.

It is better to use some unique range to avoid conflicts and
extra configuration of network environment.

Fixes: https://tracker.ceph.com/issues/70817
Signed-off-by: Kyr Shatskyy <kyrylo.shatskyy@clyso.com>