Fix integer overflow in block migration bandwidth calculation

block_mig_state.reads is an int, and multiplying by BLOCK_SIZE yielded a
negative number, resulting in a negative bandwidth (running on a 32-bit
machine).  Change order to avoid.

Signed-off-by: Avishay Traeger <avishay@il.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>