net: fold __skb_checksum() into skb_checksum()

net: fold __skb_checksum() into skb_checksum()

Now that the only remaining caller of __skb_checksum() is
skb_checksum(), fold __skb_checksum() into skb_checksum().  This makes
struct skb_checksum_ops unnecessary, so remove that too and simply do
the "regular" net checksum.  It also makes the wrapper functions
csum_partial_ext() and csum_block_add_ext() unnecessary, so remove those
too and just use the underlying functions.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Link: https://patch.msgid.link/20250519175012.36581-7-ebiggers@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>