mgr: register OSDs in ms_handle_accept

mgr: register OSDs in ms_handle_accept

It's a no-no to acquire locks in these "fast" messenger methods. This
can lead to messenger slow downs in the best case as it's blocking reads
on the wire. In the worse case, the messenger may deadlock with other
threads, preventing any further message reads off the wire.

It's not obvious this method is "fast" so I've added a comment regarding
this.

Fixes: https://tracker.ceph.com/issues/61874
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>
(cherry picked from commit 69980823e62f67d502c4045e15c41c5c44cd5127)