exfat: add exfat_get_dentry_set_by_ei() helper

This helper gets the directory entry set of the file for the exfat
inode which has been created.

It's used to remove all the instances of the pattern it replaces
making the code cleaner, it's also a preparation for changing ->dir
to record the cluster where the directory entry set is located and
changing ->entry to record the index of the directory entry within
the cluster.

Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Reviewed-by: Aoyama Wataru <wataru.aoyama@sony.com>
Reviewed-by: Daniel Palmer <daniel.palmer@sony.com>
Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>