neighbor: Initialize protocol when new pneigh_entry are created

pneigh_lookup uses kmalloc versus kzalloc when new entries are allocated.
Given that the newly added protocol field needs to be initialized.

Fixes: df9b0e30 ("neighbor: Add protocol attribute")
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>