osd: allow pg log trim during (non-classic) scrub

Chunky (and deep) scrub do not care about PG log trimming.  Classic scrub
still does.

Deep scrub can take a long time, so not trimming the log during that period
may eat lots of RAM; avoid that!

Might fix: #4179
Signed-off-by: Sage Weil <sage@inktank.com>