btrfs.c: simplify BTRFS_IOC_GET_FSLABEL parser

* btrfs.c (btrfs_ioctl): Share code of parser of BTRFS_IOC_SET_FSLABEL
with parser of BTRFS_IOC_GET_FSLABEL only on BTRFS_IOC_GET_FSLABEL's
entering.
diff --git a/btrfs.c b/btrfs.c
index 4c02b89..dd0ae22 100644
--- a/btrfs.c
+++ b/btrfs.c
@@ -1331,11 +1331,13 @@
 		tprintf("{transid=%" PRI__u64 "}", args.transid);
 		break;
 	}
+
 	case BTRFS_IOC_GET_FSLABEL: /* R */
-	case BTRFS_IOC_SET_FSLABEL: {/* W */
-		char label[BTRFS_LABEL_SIZE];
-		if (code == BTRFS_IOC_GET_FSLABEL && entering(tcp))
+		if (entering(tcp))
 			return 0;
+		/* fall through */
+	case BTRFS_IOC_SET_FSLABEL: { /* W */
+		char label[BTRFS_LABEL_SIZE];
 
 		tprints(", ");
 		if (umove_or_printaddr(tcp, arg, &label))