erofs: fix uninitialized page cache reported by KMSAN

syzbot reports a KMSAN reproducer [1] which generates a crafted
filesystem image and causes IMA to read uninitialized page cache.

Later, (rq->outputsize > rq->inputsize) will be formally supported
after either large uncompressed pclusters (> block size) or big
lclusters are landed.  However, currently there is no way to generate
such filesystems by using mkfs.erofs.

Thus, let's mark this condition as unsupported for now.

[1] https://lore.kernel.org/r/0000000000002be12a0611ca7ff8@google.com

Reported-and-tested-by:  <syzbot+7bc44a489f0ef0670bd5@syzkaller.appspotmail.com>
Fixes: 1ca01520

 ("erofs: refine z_erofs_transform_plain() for sub-page block support")
Reviewed-by: Sandeep Dhavale <dhavale@google.com>
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240304035339.425857-1-hsiangkao@linux.alibaba.com