mds: be more careful about decoding LogEvents

We need to wrap the full decode section or we can abort the process
if there's an issue (which we may want to just skip by).

Signed-off-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
Merge branch 'master' of github.com:ceph/ceph