netfilter: xt_recent: fix proc-file addition/removal of IPv4 addresses

Fix regression introduded by commit 079aa88f

 (netfilter: xt_recent: IPv6 support):

From http://bugzilla.kernel.org/show_bug.cgi?id=12753:

Problem Description:
An uninitialized buffer causes IPv4 addresses added manually (via the +IP
command to the proc interface) to never match any packets. Similarly, the -IP
command fails to remove IPv4 addresses.

Details:
In the function recent_entry_lookup, the xt_recent module does comparisons of
the entire nf_inet_addr union value, both for IPv4 and IPv6 addresses. For
addresses initialized from actual packets the remaining 12 bytes not occupied
by the IPv4 are zeroed so this works correctly. However when setting the
nf_inet_addr addr variable in the recent_mt_proc_write function, only the IPv4
bytes are initialized and the remaining 12 bytes contain garbage.

Hence addresses added in this way never match any packets, unless these
uninitialized 12 bytes happened to be zero by coincidence. Similarly, addresses
cannot consistently be removed using the proc interface due to mismatch of the
garbage bytes (although it will sometimes work to remove an address that was
added manually).

Reading the /proc/net/xt_recent/ entries hides this problem because this only
uses the first 4 bytes when displaying IPv4 addresses.

Steps to reproduce:
$ iptables -I INPUT -m recent --rcheck -j LOG
$ echo +169.254.156.239 > /proc/net/xt_recent/DEFAULT
$ cat /proc/net/xt_recent/DEFAULT
src=169.254.156.239 ttl: 0 last_seen: 119910 oldest_pkt: 1 119910

[At this point no packets from 169.254.156.239 are being logged.]

$ iptables -I INPUT -s 169.254.156.239 -m recent --set
$ cat /proc/net/xt_recent/DEFAULT
src=169.254.156.239 ttl: 0 last_seen: 119910 oldest_pkt: 1 119910
src=169.254.156.239 ttl: 255 last_seen: 126184 oldest_pkt: 4 125434, 125684, 125934, 126184

[At this point, adding the address via an iptables rule, packets are being
logged correctly.]

$ echo -169.254.156.239 > /proc/net/xt_recent/DEFAULT
$ cat /proc/net/xt_recent/DEFAULT
src=169.254.156.239 ttl: 0 last_seen: 119910 oldest_pkt: 1 119910
src=169.254.156.239 ttl: 255 last_seen: 126992 oldest_pkt: 10 125434, 125684, 125934, 126184, 126434, 126684, 126934, 126991, 126991, 126992
$ echo -169.254.156.239 > /proc/net/xt_recent/DEFAULT
$ cat /proc/net/xt_recent/DEFAULT
src=169.254.156.239 ttl: 0 last_seen: 119910 oldest_pkt: 1 119910
src=169.254.156.239 ttl: 255 last_seen: 126992 oldest_pkt: 10 125434, 125684, 125934, 126184, 126434, 126684, 126934, 126991, 126991, 126992

[Removing the address via /proc interface failed evidently.]

Possible solutions:
- initialize the addr variable in recent_mt_proc_write
- compare only 4 bytes for IPv4 addresses in recent_entry_lookup

Signed-off-by: Patrick McHardy <kaber@trash.net>