osd: get fsid from monmap, not osdmap

osd: get fsid from monmap, not osdmap

We may not have a valid OSDMap in all of these cases (notably, during
boot).  Always take the fsid from the monmap, which will be valid after
we've authenticated.

This fixes messages like

2011-12-29 08:53:44.530830 7ff3595e2700 mon.a@0(leader).pg v5 handle_statfs on fsid 00000000-0000-0000-0000-000000000000 != f8a6383d-5fbe-4f65-907e-f8d09e1d540d

on the monitor from MPGStats messages with a bad fsid right after osd boot.

Signed-off-by: Sage Weil <sage@newdream.net>