netfilter: conntrack: remove sprintf usage

netfilter: conntrack: remove sprintf usage

Replace it with scnprintf, the buffer sizes are expected to be large enough
to hold the result, no need for snprintf+overflow check.

Increase buffer size in mangle_content_len() while at it.

BUG: KASAN: stack-out-of-bounds in vsnprintf+0xea5/0x1270
Write of size 1 at addr [..]
 vsnprintf+0xea5/0x1270
 sprintf+0xb1/0xe0
 mangle_content_len+0x1ac/0x280
 nf_nat_sdp_session+0x1cc/0x240
 process_sdp+0x8f8/0xb80
 process_invite_request+0x108/0x2b0
 process_sip_msg+0x5da/0xf50
 sip_help_tcp+0x45e/0x780
 nf_confirm+0x34d/0x990
 [..]

Fixes: 9fafcd7b2032 ("[NETFILTER]: nf_conntrack/nf_nat: add SIP helper port")
Reported-by: Yiming Qian <yimingqian591@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>