mtd: block2mtd: add support for an optional custom MTD label

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