net: checksum: Move from32to16() to generic header

from32to16() is used by lib/checksum.c and also by
arch/parisc/lib/checksum.c. The next patch will use it in the
bpf_csum_diff helper.

Move from32to16() to the include/net/checksum.h as csum_from32to16() and
remove other implementations.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20241026125339.26459-2-puranjay@kernel.org