net: Protect net_state from reentrant net_loop()

net: Protect net_state from reentrant net_loop()

Global variable "net_state" is used in net_loop() state-machine.
But it happens that some times the net_loop() can be called
multiple times in the same call stack. For example when the
netconsole is enabled and we print the message while some other
net protocol is in action. Netconsole will overwrite the "net_state"
and that will break the logic for earlier started protocol.

To protect the state save and restore "net_state" variable each
time when we enter and exit net_loop().

Signed-off-by: Leonid Iziumtsev <leonid.iziumtsev@se.atlascopco.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>