FileStore: report l_os_j_lat as commit latency

l_os_commit_lat is actually the commit cycle latency.

Fixes: #9269
Backport: firefly
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #2454 from athanatos/wip-9269

FileStore: report l_os_j_lat as commit latency

Reviewed-by: Sage Weil <sage@redhat.com>