mtd: nftl: reduce stack usage in NFTL_movebuf()

The code in the ntfl write function is rather complex, and it contains
a 512 byte on-stack buffer. The combination of these two leads to using
more than the per-function stack warning limit in some configurations,
especially with KASAN enabled:

drivers/mtd/nftlcore.c:673:12: error: stack frame size (1328) exceeds limit (1280) in 'nftl_writeblock' [-Werror,-Wframe-larger-than]

Avoid this warning by moving the on-stack buffer into a separate function
that only copies one part of the device to another.

This does not really help with the total maximum stack usage in the
(non-KASAN) normal case, but it does two things:

 - no single function has more than the warning limit
 - the complexity goes down, so the parent function ends up
   spilling few local variables, and the total actually goes
   down slightly.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>