arp: fixed -Wuninitialized compiler warning

Commit 7d472a59 ("arp: always override
existing neigh entries with gratuitous ARP") introduced a compiler
warning:

net/ipv4/arp.c:880:35: warning: 'addr_type' may be used uninitialized in
this function [-Wmaybe-uninitialized]

While the code logic seems to be correct and doesn't allow the variable
to be used uninitialized, and the warning is not consistently
reproducible, it's still worth fixing it for other people not to waste
time looking at the warning in case it pops up in the build environment.
Yes, compiler is probably at fault, but we will need to accommodate.

Fixes: 7d472a59 ("arp: always override existing neigh entries with gratuitous ARP")
Signed-off-by: Ihar Hrachyshka <ihrachys@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>