ObjectCacher: use uint64_t for target and max values

ObjectCacher: use uint64_t for target and max values

All the options are uint64_t, but the ObjectCacher was converting them
to int64_t. There's never any reason for these to be negative, so
change the type.

Adjust a few conditionals so that they only convert known-positive
signed values to uint64_t before comparing with the target and max
values. Leave the actual stats accounting as loff_t for now, since
bugs in accounting will have bad effects if negative values wrap
around.

Backport: emperor, dumpling
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>