cache invalidation error for buffered write

The bug causes corruptions of data read from flash.

The original code performs cache invalidation from "adr" to "adr + len"
in do_write_buffer().  Since len and adr could be updated in the code
before invalidation - it causes improper setting of cache invalidation
regions.

Signed-off-by: Massimo Cirillo <maxcir@gmail.com>
Signed-off-by: Giuseppe D'Eliseo <giuseppedeliseo@gmail.com>
Acked-by: Nicolas Pitre <nico@cam.org>
Acked-by: Jörn Engel <joern@logfs.org>
Signed-off-by: David Woohouse <dwmw2@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>