Decompressors: fix callback-to-callback mode in decompress_unlzo.c

Callback-to-callback decompression mode is used for initrd (not
initramfs).  The LZO wrapper is broken for this use case for two reasons:

  - The argument validation is needlessly too strict by
    requiring that "posp" is non-NULL when "fill" is non-NULL.

  - The buffer handling code didn't work at all for this
    use case.

I tested with LZO-compressed kernel, initramfs, initrd, and corrupt
(truncated) initramfs and initrd images.

Signed-off-by: Lasse Collin <lasse.collin@tukaani.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Alain Knaff <alain@knaff.lu>
Cc: Albin Tonnerre <albin.tonnerre@free-electrons.com>
Cc: Phillip Lougher <phillip@lougher.demon.co.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>