btrfs: send: replace check with an assert in gen_unique_name

The buffer passed to snprintf can hold the fully expanded format string,
64 = 3x largest ULL + 3x char + trailing null.  I don't think that removing the
check entirely is a good idea, hence the ASSERT.
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fb.com>

fs/btrfs/send.c

@@ -1418,11 +1418,7 @@ static int gen_unique_name(struct send_ctx *sctx,
 	while (1) {
 		len = snprintf(tmp, sizeof(tmp), "o%llu-%llu-%llu",
 				ino, gen, idx);
-		if (len >= sizeof(tmp)) {
-			/* should really not happen */
-			ret = -EOVERFLOW;
-			goto out;
-		}
+		ASSERT(len < sizeof(tmp));
 
 		di = btrfs_lookup_dir_item(NULL, sctx->send_root,
 				path, BTRFS_FIRST_FREE_OBJECTID,