inet_getpeer_v4() can return NULL under OOM conditions, and while
inet_peer_xrlim_allow() is OK with a NULL peer, inet_putpeer() will
crash.

This code path now uses the same idiom as the others from:
1d861aa4 ("inet: Minimize use of
cached route inetpeer.").
Signed-off-by: NNeal Cardwell <ncardwell@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>