mds: update segment references during journal rewrite

... to avoid leaving log events that reference log
segments by offsets which no longer exist.

Signed-off-by: John Spray <john.spray@redhat.com>
Merge pull request #2469 from ceph/wip-9427-rewrite

mds: update segment references during journal rewrite

Reviewed-by: Greg Farnum <greg@inktank.com>