libceph: avoid using freed osd in __kick_osd_requests()

If an osd has no requests and no linger requests, __reset_osd()
will just remove it with a call to __remove_osd().  That drops
a reference to the osd, and therefore the osd may have been free
by the time __reset_osd() returns.  That function offers no
indication this may have occurred, and as a result the osd will
continue to be used even when it's no longer valid.

Change__reset_osd() so it returns an error (ENODEV) when it
deletes the osd being reset.  And change __kick_osd_requests() so it
returns immediately (before referencing osd again) if __reset_osd()
returns *any* error.
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>

net/ceph/osd_client.c

@@ -581,7 +581,7 @@ static void __kick_osd_requests(struct ceph_osd_client *osdc,
 
 	dout("__kick_osd_requests osd%d\n", osd->o_osd);
 	err = __reset_osd(osdc, osd);
-	if (err == -EAGAIN)
+	if (err)
 		return;
 
 	list_for_each_entry(req, &osd->o_requests, r_osd_item) {
@@ -745,6 +745,7 @@ static int __reset_osd(struct ceph_osd_client *osdc, struct ceph_osd *osd)
 	if (list_empty(&osd->o_requests) &&
 	    list_empty(&osd->o_linger_requests)) {
 		__remove_osd(osdc, osd);
+		ret = -ENODEV;
 	} else if (memcmp(&osdc->osdmap->osd_addr[osd->o_osd],
 			  &osd->o_con.peer_addr,
 			  sizeof(osd->o_con.peer_addr)) == 0 &&