osd: drop writes when full instead of returning an error

There's a race between the client and osd with a newly marked full
osdmap.  If the client gets the new map first, it blocks writes and
everything works as expected, with no errors from the osd.

If the osd gets the map first, however, it will respond to any writes
with -ENOSPC. Clients will pass this up the stack, and not retry these
writes later.  -ENOSPC isn't handled well by all clients. RBD, for
example, may pass it on to qemu or kernel rbd which will both
interpret it as EIO.  Filesystems on top of rbd will not behave well
when they receive EIOs like this, especially if the cluster oscillates
between full and not full, so some writes succeed.

To fix this, never return ENOSPC from the osd because of a map marked
full, and rely on the client to retry all writes when the map is no
longer marked full.

Old clients talking to osds with this fix will hang instead of
propagating an error, but only if they run into this race
condition. ceph-fuse and rbd with caching enabled are not affected,
since the ObjectCacher will retry writes that return errors.

Refs: #6938
Backport: dumpling, emperor
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
objecter: resend all writes after osdmap loses the full flag

Now that the osd does not respond if it gets a map with the full flag
set first, clients need to resend all writes.

Clients talking to old osds are still subject to the race condition,
so both sides must be upgraded to avoid it.

Refs: #6938
Backport: dumpling, emperor
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
rbd: check write return code during bench-write

This is allows rbd-bench to detect http://tracker.ceph.com/issues/6938
when combined with rapidly changing the mon osd full ratio.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>