9p: fix a race condition bug in umount which caused a segfault

umounting partitions after heavy activity would sometimes trigger a
segmentation violation.  This fix appears to remove that problem.
Fix originally provided by Latchesar Ionkov.
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

net/9p/client.c

@@ -127,12 +127,12 @@ void p9_client_destroy(struct p9_client *clnt)
 		clnt->trans = NULL;
 	}
 
-	if (clnt->fidpool)
-		p9_idpool_destroy(clnt->fidpool);
-
 	list_for_each_entry_safe(fid, fidptr, &clnt->fidlist, flist)
 		p9_fid_destroy(fid);
 
+	if (clnt->fidpool)
+		p9_idpool_destroy(clnt->fidpool);
+
 	kfree(clnt);
 }
 EXPORT_SYMBOL(p9_client_destroy);