mtd: Add sysfs attribute for mtd OOB available size

Expose mtd OOB available size by sysfs file. Then users can get available
OOB size by accessing /sys/class/mtd/mtdX/oobavail.

Signed-off-by: Xiaolei Li <[e-mail]>
Signed-off-by: Boris Brezillon <[e-mail]>

Documentation/ABI/testing/sysfs-class-mtd

···
 		of the parent (another partition or a flash device) in bytes.
 		This attribute is absent on flash devices, so it can be used
 		to distinguish them from partitions.
+
+What:		/sys/class/mtd/mtdX/oobavail
+Date:		April 2018
+KernelVersion:	4.16
+Contact:	[e-mail]
+Description:
+		Number of bytes available for a client to place data into
+		the out of band area.

drivers/mtd/mtdcore.c

···
 }
 static DEVICE_ATTR(oobsize, S_IRUGO, mtd_oobsize_show, NULL);
 
+static ssize_t mtd_oobavail_show(struct device *dev,
+				 struct device_attribute *attr, char *buf)
+{
+	struct mtd_info *mtd = dev_get_drvdata(dev);
+
+	return snprintf(buf, PAGE_SIZE, "%u\n", mtd->oobavail);
+}
+static DEVICE_ATTR(oobavail, S_IRUGO, mtd_oobavail_show, NULL);
+
 static ssize_t mtd_numeraseregions_show(struct device *dev,
 		struct device_attribute *attr, char *buf)
 {
···
 	&dev_attr_writesize.attr,
 	&dev_attr_subpagesize.attr,
 	&dev_attr_oobsize.attr,
+	&dev_attr_oobavail.attr,
 	&dev_attr_numeraseregions.attr,
 	&dev_attr_name.attr,
 	&dev_attr_ecc_strength.attr,