libceph: pick a different monitor when reconnecting

Don't try to reconnect to the same monitor when we fail to establish
a session within a timeout or it's lost.

For that, pick_new_mon() needs to see the old value of cur_mon, so
don't clear it in __close_session() - all calls to __close_session()
but one are followed by __open_session() anyway.  __open_session() is
only called when a new session needs to be established, so the "already
open?" branch, which is now in the way, is simply dropped.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>