exfat: fix exfat_find_empty_entry() not returning error on failure

On failure, "dentry" is the error code. If the error code indicates
that there is no space, a new cluster may need to be allocated; for
other errors, it should be returned directly.

Only on success, "dentry" is the index of the directory entry, and
it needs to be converted into the directory entry index within the
cluster where it is located.

Fixes: 8a3f5711 ("exfat: reduce FAT chain traversal")
Reported-by:  <syzbot+6f6c9397e0078ef60bce@syzkaller.appspotmail.com>
Tested-by:  <syzbot+6f6c9397e0078ef60bce@syzkaller.appspotmail.com>
Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>