osd: fix filestore perf stats update

Update the struct we are about to send, not the (unlocked!) one we will
send the next time around.

Backport: firefly, dumpling
Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 4afffb4a10a0bbf7f2018ef3ed6b167c7921e46b)