utils: Introduce and use inet_prefix_reset()

utils: Introduce and use inet_prefix_reset()

Initializing @inet_prefix using C initializers or memset() seems
inefficient and unnecessary: only small part of ->data[] field will be
used to store address corresponding to ->family.

Instead initialize ->flags with zero and assume no other fields accessed
before checking corresponding bits in ->flags. For example special
helpers (e.g. is_addrtype_*()) can be used to ensure that @inet_prefix
contains valid ip or ipv6 address.

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: David Ahern <dsahern@gmail.com>