Hyper-V: hn: rewrite hn rsc swtich to avoid sysctl hang

Changing the rsc_switch flag using sysctl to turn rsc on or off
could hang. The orignal code sends request to host to get the
mtu setting. Sometimes the host fails to reply, causing
the thread to sleep forever waiting for the host response.

Use existing cached mtu from hn device instead to avoid calling
host.

Reported by: whu
Tested by: whu
MFC after: 1 week

(cherry picked from commit da1deb784d9ad3a4015a3f91fa1a5ce394fd3fdb)