osd/ReplicatedPG: fix trim of in-flight hit_sets

We normally need to stat the hit_set to know how many bytes to adjust the 
stats by.  If the hit_set was just written, we will get ENOENT.  Augment the
hit_set_flushing map<> with a size map so that we can skip the stat entirely
for recent hit_sets.

Fixes: #8283 Backport: firefly Signed-off-by: Sage Weil <sage@inktank.com>