mtd: mtdoops: Create a header structure for the saved mtdoops.

Create a dump header to enable the addition of fields without having
to modify the rest of the code.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Jean-Marc Eurin <jmeurin@google.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220421234244.2172003-3-jmeurin@google.com