Fix info unit bytes suffix

[gerhard]

B is used for powers of 10, e.g. 10^6 bytes = 1MB
iB is used for powers of 2, e.g. 2^20 bytes = 1MiB

Since the division is using a power of 2, the symbol used also needs to
be a power of 2.

Types of Changes

What types of changes does your code introduce to this project?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (correction or otherwise)
• Cosmetics (whitespace, appearance)

Checklist

• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• All tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in related repositories

Further Comments

This is somewhat related to rabbitmq/rabbitmq-server#1987

Before this change, 93048832 bytes were shown as 89MB. After this change, they are shown as 89MiB. This applies to all units:

[michaelklishin]

Backported to v3.7.x.