zebra: fix detection of VRF misconfiguration

zebra: fix detection of VRF misconfiguration

The netlink_vrf_change() function is called both when a VRF device
is created in the Linux kernel and when it is activated. This
commit changes this function to perform the VRF misconfiguration
detection only when the VRF device is created, as doing the check
twice would cause a false positive followed by a hard failure (not
to mention the double check is unnecessary since the VRF table ID
can't change once the device is created).

Fixes #6319.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>