[NET]: XTENSA checksum annotations and cleanups.

* sanitize prototypes, annotate
* kill csum_partial_copy_fromuser
* kill csum_partial_copy
* kill useless shifts
* usual ntohs->shift

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