erofs: don't trigger WARN() when decompression fails

syzbot reported a WARNING [1] due to corrupted compressed data.

As Dmitry said, "If this is not a kernel bug, then the code should
not use WARN. WARN if for kernel bugs and is recognized as such by
all testing systems and humans."

[1] https://lore.kernel.org/r/000000000000b3586105cf0ff45e@google.com

Link: https://lore.kernel.org/r/20211025074311.130395-1-hsiangkao@linux.alibaba.com


Cc: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Reported-by:  <syzbot+d8aaffc3719597e8cfb4@syzkaller.appspotmail.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>