rbd: Fix the rbd export when image size more than 2G

When using export <image-name> <path> and the size of image is more than 2G,
the previous version about finish() could not handle in seeking the offset
in image and return error.

This is caused by the incorrect variable type. Try to use the correct
variable type to fixed it.

I use another vailable which type is uint64_t for confirming seeking and
still use the previous r for return error.

uint64_t is more better than type int for handle lseek64().

Signed-off-by: Vicente Cheng <freeze.bilsted@gmail.com>