flat: fix unmap len in load error path

The data chunk is mmaped with 'len' which remains unchanged, so use that
when unmapping in the error path rather than trying to recalculate (and
incorrectly so) the value used originally.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: David McCullough <davidm@snapgear.com>
Acked-by: Greg Ungerer <gerg@uclinux.org>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Hirokazu Takata <takata@linux-m32r.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>