erofs: fix null-ptr-deref caused by erofs_xattr_prefixes_init

Fragments and dedupe share one feature bit, and thus packed inode may not
exist when fragment feature bit (dedupe feature bit exactly) is set, e.g.
when deduplication feature is in use while fragments feature is not.  In
this case, sbi->packed_inode could be NULL while fragments feature bit
is set.

Fix this by accessing packed inode only when it exists.

Reported-by: syzbot+902d5a9373ae8f748a94@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?extid=902d5a9373ae8f748a94
Reported-and-tested-by: syzbot+bbb353775d51424087f2@syzkaller.appspotmail.com
Fixes: 9e38291 ("erofs: add helpers to load long xattr name prefixes")
Fixes: 6a318cc ("erofs: enable long extended attribute name prefixes")
Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20230515103941.129784-1-jefflexu@linux.alibaba.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

fs/erofs/xattr.c

@@ -675,7 +675,7 @@ int erofs_xattr_prefixes_init(struct super_block *sb)
 	if (!pfs)
 		return -ENOMEM;
 
-	if (erofs_sb_has_fragments(sbi))
+	if (sbi->packed_inode)
 		buf.inode = sbi->packed_inode;
 	else
 		erofs_init_metabuf(&buf, sb);