[NET]: Alpha checksum annotations and cleanups.

* sanitize prototypes and annotate
* kill useless access_ok() in csum_partial_copy_from_user() (the only
caller checks it already).
* do_csum_partial_copy_from_user() is not needed now
* replace htons(len) with len << 8 - they are the same wrt checksums
on little-endian.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>