zebra: Don't leak memory in rtadv.c code on shutdown

zebra: Don't leak memory in rtadv.c code on shutdown

The adata variable was being leaked on shutdown since
it was calloc'ed.  There is no need to make this dynamic
memory.  Just choose a size and use that.  Add a bit
of code to ensure that if it's not large enough,
it will just stop and the developer will fix it.

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