ipv6: default IPV6_SIT to m

ipv6: default IPV6_SIT to m

This basically defaulted to m until recently, since IPV6 defaulted to
m.  Since IPV6 was changed to a boolean with a default of y, IPV6_SIT
started defaulting to built-in as well.  This results in a surprise
sit0 device by default for defconfig (and defconfig-derived config)
users at boot.  For me, this broke an (admittedly non-robust) script.
Preserve the behaviour of most configs by avoiding building this
module, that's probably overall seldom used compared to IPv6 as a
whole, into the kernel.

Fixes: 309b905deee59 ("ipv6: convert CONFIG_IPV6 to built-in only and clean up Kconfigs")
Signed-off-by: Alyssa Ross <hi@alyssa.is>
Reviewed-by: Fernando Fernandez Mancera <fmancera@suse.de>
Link: https://patch.msgid.link/20260503192515.290900-2-hi@alyssa.is
Signed-off-by: Jakub Kicinski <kuba@kernel.org>