mtd: mtdpart: Convert sysfs sprintf/snprintf family to sysfs_emit

Use sysfs_emit instead of snprintf to avoid buf overrun,because in
sysfs_emit it strictly checks whether buf is null or buf whether
pagesize aligned, otherwise it returns an error.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/1618220144-33839-3-git-send-email-tiantao6@hisilicon.com