osd: fix bad Message* defer in C_SendMap and send_map_on_destruct

We were carrying a bare Message*, which could get freed if the op was
canceled (or possibly completed).  Instead, just stash the entity_name_t,
the only piece we need.  The Connection is properly ref counted so no
worries there.

Fixes: #8926
Signed-off-by: Sage Weil <sage@redhat.com>