storage_backend_rbd: Correct argument order to rbd_create3
The stripe_unit and stripe_count arguments are passed to rbd_create3 in
the wrong order, resulting in a stripe size of 1 byte with 4194304
stripes on newly created RBD volumes.
https://bugzilla.redhat.com/show_bug.cgi?id=1092208
Signed-off-by: Steven McDonald <steven.mcdonald@anchor.net.au>