lib/repo: Ensure min-free-space* config value doesn't overflow

lib/repo: Ensure min-free-space* config value doesn't overflow
when converted to bytes

In a subsequent commit, we add a public API to read the value of
min-free-space-* value in bytes. The value for free space check
is enforced in terms of block size instead of bytes. Therefore,
for consistency we check while preparing the transaction that the
value doesn't overflow when converted to bytes.

https://phabricator.endlessm.com/T23694

Closes: #1715
Approved by: cgwalters