pimd: Warn when we try to build MAXVIFS > 256

pimd: Warn when we try to build MAXVIFS > 256

We use the pim mroute socket for kernel notifications of events.
Currently this is limited to 8 bits of data.  There are patches
coming down the pike in kernel land to allow this to expand.
Rather than fix this and all the other places we assume MAXVIFS < 256
in the pim code right now.  Leave a land mine for the developer
doing this work to point them in the right direction.

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