Fix RBD store to use READ_CHUNKSIZE

Change 4e0c563b8f3a5ced8f65fcca83d341a97729a5d4 was incomplete and
missed a variable in the rbd store.

This patch fixes the issue by using store.READ_CHUNKSIZE instead of
store.chunk_size

Change-Id: I2dc5b1514d3e86ec129c1fc88de096d98a64f806
Closes-Bug: #1352509