osd: trim pg logs based on a per-osd budget

osd: trim pg logs based on a per-osd budget

Set the default budget based on the current defaults: 3000 per osd, and a
rule of thumb target of 100 PGs per OSD.  Set the per-PG trim target
by dividing the overall value by the number of PGs on the OSD.

Increase the max pg log length alone, so if the OSD has <100 PGs,
those PGs will get more entries.  Reduce the minimum to be smaller than
the max.  Use the min/max config options to bracket what is allocated to
a single PG.

Signed-off-by: Sage Weil <sage@redhat.com>