exfat: fix uninit-value in __exfat_get_dentry_set

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>