libceph: allocate ceph_osd with GFP_NOFAIL

create_osd() is called way too deep in the stack to be able to error
out in a sane way; a failing create_osd() just messes everything up.
The current req_notarget list solution is broken - the list is never
traversed as it's not entirely clear when to do it, I guess.

If we were to start traversing it at regular intervals and retrying
each request, we wouldn't be far off from what __GFP_NOFAIL is doing,
so allocate OSD sessions with __GFP_NOFAIL, at least until we come up
with a better fix.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

net/ceph/osd_client.c

@@ -1043,10 +1043,7 @@ static struct ceph_osd *create_osd(struct ceph_osd_client *osdc, int onum)
 
 	WARN_ON(onum == CEPH_HOMELESS_OSD);
 
-	osd = kzalloc(sizeof(*osd), GFP_NOFS);
-	if (!osd)
-		return NULL;
-
+	osd = kzalloc(sizeof(*osd), GFP_NOIO | __GFP_NOFAIL);
 	osd_init(osd);
 	osd->o_osdc = osdc;
 	osd->o_osd = onum;