UBIFS: correct mount message

When mounting an UBIFS R/W volume, we have the message:
UBIFS: mounted UBI device 0, volume 1, name "rootfs"(null)
With this patch, we'll have:
UBIFS: mounted UBI device 0, volume 1, name "rootfs"
Which is, I think, what was intended.
Signed-off-by: NRichard Genoud <richard.genoud@gmail.com>
Cc: stable@vger.kernel.org [v3.7+]
Signed-off-by: NArtem Bityutskiy <artem.bityutskiy@linux.intel.com>

fs/ubifs/super.c

@@ -1412,7 +1412,7 @@ static int mount_ubifs(struct ubifs_info *c)
 
 	ubifs_msg("mounted UBI device %d, volume %d, name \"%s\"%s",
 		  c->vi.ubi_num, c->vi.vol_id, c->vi.name,
-		  c->ro_mount ? ", R/O mode" : NULL);
+		  c->ro_mount ? ", R/O mode" : "");
 	x = (long long)c->main_lebs * c->leb_size;
 	y = (long long)c->log_lebs * c->leb_size + c->max_bud_bytes;
 	ubifs_msg("LEB size: %d bytes (%d KiB), min./max. I/O unit sizes: %d bytes/%d bytes",