[Blackfin] arch: Blackfin checksum annotations

FSVOtest in this case, since I don't have the hardware...
However, all changes seen by gcc are actually
 - explicit cast to unsigned short in return expression of functions
    returning unsigned short
 - csum_fold() return type changed from unsigned int to __sum16
   (unsigned short), same as for all other architecture and as net/* expects;
   expression actually returned is ((~(sum << 16)) >> 16) with sum being
   unsigned 32bit, so it's (a) going to fit into the range of unsigned short
   and (b) had been unsigned all along, so no sign expansion mess happened.

Tested-by: Bryan Wu <cooloney@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David Miller <davem@davemloft.net>
Signed-off-by: Bryan Wu <cooloney@kernel.org>