Fix RBD store to use READ_CHUNKSIZE and correct return of get()

1. Fix reading chunksize of RBD store.
2. Correct return value of get() interface of RBD store.

Change-Id: Idd236d290234e49edf5f665604b0e6861663d4d8
Related-Id: I2dc5b1514d3e86ec129c1fc88de096d98a64f806
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>