osd: compute full ratio from kb_avail

On btrfs, kb_used + kb_avail can be much smaller than total kb, and
what really matters to avoid filling up the disk is how much space is
available, not how much we've used.  Thus, compute the ratio we use to
determine full or nearfull from kb_avail rather than from kb_used.

Signed-off-by: Alexandre Oliva <oliva@gnu.org>
Signed-off-by: Sage Weil <sage@inktank.com>