exfat: fix uninit-value in __exfat_get_dentry_set

There is no check if stream size and start_clu are invalid.
If start_clu is EOF cluster and stream size is 4096, It will
cause uninit value access. because ei->hint_femp.eidx could
be 128(if cluster size is 4K) and wrong hint will allocate
next cluster. and this cluster will be same with the cluster
that is allocated by exfat_extend_valid_size(). The previous
patch will check invalid start_clu, but for clarity, initialize
hint_femp.eidx to zero.

Cc: stable@vger.kernel.org
Reported-by:  <syzbot+01218003be74b5e1213a@syzkaller.appspotmail.com>
Tested-by:  <syzbot+01218003be74b5e1213a@syzkaller.appspotmail.com>
Reviewed-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>

fs/exfat/namei.c

@@ -345,6 +345,7 @@ static int exfat_find_empty_entry(struct inode *inode,
 		if (ei->start_clu == EXFAT_EOF_CLUSTER) {
 			ei->start_clu = clu.dir;
 			p_dir->dir = clu.dir;
+			hint_femp.eidx = 0;
 		}
 
 		/* append to the FAT chain */