Fix comparison signedness in arc_is_overflowing().

When ARC size is very small, aggsum_lower_bound(&arc_size) may return
negative values, that due to unsigned comparison caused delays, waiting
for arc_adjust() to "fix" it by calling aggsum_value(&arc_size). Use
of signed comparison there fixes the problem.

MFC after: 2 weeks
Sponsored by: iXsystems, Inc.