regmap: debugfs: Avoid overflows for very small reads

If count is less than the size of a register then we may hit integer
wraparound when trying to move backwards to check if we're still in
the buffer. Instead move the position forwards to check if it's still
in the buffer, we are unlikely to be able to allocate a buffer
sufficiently big to overflow here.
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: stable@vger.kernel.org

drivers/base/regmap/regmap-debugfs.c

@@ -93,7 +93,7 @@ static ssize_t regmap_read_debugfs(struct regmap *map, unsigned int from,
 		/* If we're in the region the user is trying to read */
 		if (p >= *ppos) {
 			/* ...but not beyond it */
-			if (buf_pos >= count - 1 - map->debugfs_tot_len)
+			if (buf_pos + 1 + map->debugfs_tot_len >= count)
 				break;
 
 			/* Format the register */