mtd: rfd_ftl: add discard support

I proposed this change 16 years ago before discard was a feature in
the block layer: https://lwn.net/Articles/162776/



Now that the block layer has discard, we can finally merge this change.

Discard is also known as trim. By implementing discard, both fstrim and
the discard filesystem option can be used.

Implementing discard in the ftl means that when files are removed, there
is less data in the ftl mapping. This means less stuff to move around for
erasing and also less erasing to do; this means improved wear levelling
and improved performance.

Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210807214538.14484-3-sean@mess.org