dpif-linux: Fail earlier if OVS kernel module isn't loaded

dpif-linux: Fail earlier if OVS kernel module isn't loaded

When the kernel module isn't loaded, the bridge tries to open all the
possible minor devices, regardless.  This change first checks that there
is a major device number for Open vSwitch and only then tries to open the
minor devices.

This change also removes the assumption that there's a default Open vSwitch
major device number, since the kernel module always attempts to get a
dynamic one.  Maybe one day we'll have one...

Bug #1179