erofs: fix shift-out-of-bounds of blkszbits

syzbot generated a crafted bitszbits which can be shifted
out-of-bounds[1]. So directly print unsupported blkszbits
instead of blksize.

[1] https://lore.kernel.org/r/000000000000c72ddd05b9444d2f@google.com

Link: https://lore.kernel.org/r/20210120013016.14071-1-hsiangkao@aol.com
Reported-by:  <syzbot+c68f467cd7c45860e8d4@syzkaller.appspotmail.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>