zebra: Ensure proper decoding of netlink message

zebra: Ensure proper decoding of netlink message

As part of the kernel netlink functionality, it is
possible that a bit of nested attributes can be
passed up.  This attribute has a type value which
is stored in the lower 8 bits and in the upper 8
bits are a couple control flags that can be used.
FRR can parse this data and then just throw away
the value unless we mask off the upper 8 bits.
Let's ensure that it can be properly parsed.

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