crimson/os/seastore: derive failsafe-full threshold from osd_failsafe_full_ratio

RBMCleaner::is_storage_full() hardcoded a 3% free limit. Read the same
config classic uses for its failsafe check (osd_failsafe_full_ratio,
default 0.97) instead, treating the store as full once the free fraction
drops below (1 - ratio), so the threshold tracks the knob rather than a
magic constant.

Signed-off-by: Shai Fultheim <shai.fultheim@gmail.com>