mon: PGMonitor: available size 0 if no osds on pool's ruleset

mon: PGMonitor: available size 0 if no osds on pool's ruleset

get_rule_avail() may return < 0, which we were using blindly assuming it
would always return an unsigned value.  We would end up with weird
values if the ruleset had no osds.

Signed-off-by: Joao Eduardo Luis <joao@redhat.com>
(cherry picked from commit 8be6a6ab2aa5a000a39c73a98b11a0ab32fffa1c)