osd: fix map subscription in YOU_DIED osd_ping handler

If we have epoch X and find out we died as of epoch Y, we still want to
request X+1.  Among other things, this fixes a 'stall' if Y happens to be
the most recent map published and no new maps are generated because we will
never get anything back from our subscription.

This makes this osdmap_subscribe() caller match every other caller by
passing in current epoch + 1.

Fixes: #8002
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1608 from ceph/wip-8002

osd: fix osd map subscribe on YOU_DIED osd_ping

Reviewed-by: Samuel Just <sam.just@inktank.com>