initrd: fix lz4 decompress with initrd

During testing initrd (>2G) support, find decompress/lz4 does not work
with initrd at all.

decompress_* should support:
1. inbuf[]/outbuf[] for kernel preboot.
2. inbuf[]/flush() for initramfs
3. fill()/flush() for initrd.

in the unlz4 does not handle case 3, as input len is passed as 0, and it
failed in first try.

Fix that add one extra if (fill) checking, and get out if EOF from the
fill().

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: Kyungsik Lee <kyungsik.lee@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>