net: introduce CONFIG_NET_CRC32C

net: introduce CONFIG_NET_CRC32C

Add a hidden kconfig symbol NET_CRC32C that will group together the
functions that calculate CRC32C checksums of packets, so that these
don't have to be built into NET-enabled kernels that don't need them.

Make skb_crc32c_csum_help() (which is called only when IP_SCTP is
enabled) conditional on this symbol, and make IP_SCTP select it.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Link: https://patch.msgid.link/20250519175012.36581-2-ebiggers@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>