ipv4: Silence intentional wrapping addition

The overflow sanitizer quickly noticed what appears to have been an old
sore spot involving intended wrap around:

[   22.192362] ------------[ cut here ]------------
[   22.193329] UBSAN: signed-integer-overflow in ../arch/x86/include/asm/atomic.h:85:11
[   22.194844] 1469769800 + 1671667352 cannot be represented in type 'int'
[   22.195975] CPU: 2 PID: 2260 Comm: nmbd Not tainted 6.7.0 #1
[   22.196927] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 0.0.0 02/06/2015
[   22.198231] Call Trace:
[   22.198641]  <TASK>
[   22.198641]  dump_stack_lvl+0x64/0x80
[   22.199533]  handle_overflow+0x152/0x1a0
[   22.200382]  __ip_select_ident+0xe3/0x100

Explicitly mark ip_select_ident() as performing wrapping signed
arithmetic. Update the passed type as a u32 since that is how it is used
(it is either u16 or a literal "1" in callers, but used with a wrapping
int, so it's actually a u32). Update the comment to mention annotation
instead ofk -fno-strict-overflow, which is no longer the issue.

Cc: Jakub Kicinski <kuba@kernel.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: David Ahern <dsahern@kernel.org>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: netdev@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>