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>
(cherry picked from commit 386f2d7c829422695a1b1f41bd3f17ca3eef1f61)
Reviewed-by: Greg Farnum <greg@inktank.com>