mds: fix log trimming races

mds: fix log trimming races

trim() would iterate over segments.  It would take the *p segment, ++p,
then call try_expire().  But the _expired() function would also clean up
and (if possible) retire subsequent segments on the list if they were on
the expired list, invalidating the p iterator.

Untangle the mess by making expired segment trimming (i.e. removing from
segment list) a separate operation performed only by trim() (probably a
good idea anyway).  This keeps the iterator safe/stable.

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