block/sed: Use ssize_t on atom parsers to return errors

The short atom parser can return an errno from decoding but does not
currently return the error as a signed value. Convert all of the parsers
to ssize_t.

Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
Reviewed-by: Scott Bauer <scott.bauer@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>