This patch adds support for an optional MTD label for mtd2block emulated
MTD devices.  Useful when, e.g., testing device images using Qemu.

The following line in /etc/fstab can then be used to mount a file system
regardless if running on an embedded system, or emulated with block2mtd:

    mtd:Config  /mnt    jffs2   noatime,nodiratime      0    0

Kernel command line syntax in the emulated case:

    block2mtd.block2mtd=/dev/sda,,Config

Notice the ',,' it is the optional erase_size, which like before this
patch, defaults to PAGE_SIZE when omitted.  Hence the strlen() check.
Signed-off-by: Joachim Wiberg <troglobit@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20211009060955.552636-3-troglobit@gmail.com