iommu/omap: Fix buffer overflow in debugfs

There are two issues here:

1) The "len" variable needs to be checked before the very first write.
Otherwise if omap2_iommu_dump_ctx() with "bytes" less than 32 it is a
buffer overflow.
2) The snprintf() function returns the number of bytes that *would* have
been copied if there were enough space. But we want to know the
number of bytes which were *actually* copied so use scnprintf()
instead.

Fixes: bd4396f09a4a ("iommu/omap: Consolidate OMAP IOMMU modules")
Signed-off-by: Dan Carpenter <[e-mail]>
Reviewed-by: Robin Murphy <[e-mail]>
Reviewed-by: Laurent Pinchart <[e-mail]>
Link: https://lore.kernel.org/r/YuvYh1JbE3v+abd5@kili
Signed-off-by: Joerg Roedel <[e-mail]>

drivers/iommu/omap-iommu-debug.c

···
 		ssize_t bytes;						\
 		const char *str = "%20s: %08x\n";			\
 		const int maxcol = 32;					\
-		bytes = snprintf(p, maxcol, str, __stringify(name),	\
+		if (len < maxcol)					\
+			goto out;					\
+		bytes = scnprintf(p, maxcol, str, __stringify(name),	\
 				 iommu_read_reg(obj, MMU_##name));	\
 		p += bytes;						\
 		len -= bytes;						\
-		if (len < maxcol)					\
-			goto out;					\
 	} while (0)
 
 static ssize_t