libceph: fix legacy layout decode with pool 0

If your data pool was pool 0, ceph_file_layout_from_legacy()
transform that to -1 unconditionally, which broke upgrades.
We only want do that for a fully zeroed ceph_file_layout,
so that it still maps to a file_layout_t.  If any fields
are set, though, we trust the fl_pgpool to be a valid pool.

Fixes: 7627151 ("libceph: define new ceph_file_layout structure")
Link: http://tracker.ceph.com/issues/17825
Signed-off-by: Yan, Zheng <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

net/ceph/ceph_fs.c

@@ -34,7 +34,8 @@ void ceph_file_layout_from_legacy(struct ceph_file_layout *fl,
 	fl->stripe_count = le32_to_cpu(legacy->fl_stripe_count);
 	fl->object_size = le32_to_cpu(legacy->fl_object_size);
 	fl->pool_id = le32_to_cpu(legacy->fl_pg_pool);
-	if (fl->pool_id == 0)
+	if (fl->pool_id == 0 && fl->stripe_unit == 0 &&
+	    fl->stripe_count == 0 && fl->object_size == 0)
 		fl->pool_id = -1;
 }
 EXPORT_SYMBOL(ceph_file_layout_from_legacy);