jffs2: Fix rtime decompressor

commit b29bf7119d6bbfd04aabb8d82b060fe2a33ef890 upstream.

The fix for a memory corruption contained a off-by-one error and
caused the compressor to fail in legit cases.

Cc: Kinsey Moore <kinsey.moore@oarcorp.com>
Cc: stable@vger.kernel.org
Fixes: fe051552f5078 ("jffs2: Prevent rtime decompress memory corruption")
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>