erofs: fix general protection fault when reading fragment

As syzbot reported [1], the fragment feature sb flag is not set, so
packed_inode != NULL needs to be checked in z_erofs_read_fragment().

[1] https://lore.kernel.org/all/0000000000002e7a8905eb841ddd@google.com/

Reported-by: syzbot+3faecbfd845a895c04cb@syzkaller.appspotmail.com
Fixes: b15b2e30 ("erofs: support on-disk compressed fragments data")
Signed-off-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20221021085325.25788-1-zbestahu@gmail.comSigned-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

fs/erofs/zdata.c

@@ -660,6 +660,9 @@ static int z_erofs_read_fragment(struct inode *inode, erofs_off_t pos,
 	u8 *src, *dst;
 	unsigned int i, cnt;
 
+	if (!packed_inode)
+		return -EFSCORRUPTED;
+
 	pos += EROFS_I(inode)->z_fragmentoff;
 	for (i = 0; i < len; i += cnt) {
 		cnt = min_t(unsigned int, len - i,