zebra: Prevent awful misconfiguration in vrf's

zebra: Prevent awful misconfiguration in vrf's

Currently the linux kernel allows you to specify the same
table id -> multiple vrf's.  While I am arguing with
the kernel people about proper behavior here let's
just remove this as a possiblity from happening and
mark it a zebra stopable misconfiguration.

(Effectively we are preventing a crash down the line
as that all over FRR we assume it's a unique
mapping not a many to one).

Why fail hard?  Because we hope to get the person
who misconfigured it to actually notice immediately
not hours or days down the line when shit hits the fan.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>