Nougat: How to use Full Disk Encryption instead of File Based Encryption?
You have a couple of options - regarding device status, ROM origin &amp;&amp; course of action -  though every1 of them is focused on changing flags in fstab.qcom.

Basically, while flag in fstab.qcom for FBE is 

fileencryption=contents_aes-256-xts[:filenames_aes-256-heh]

and defines options for content &amp;&amp; metadata encryption, flag for FDE is

forceencrypt

or

encryptable

U don't need to root.

Because u have nasty Nexus 5X (had very bad experience with that device) u can unlock bootloader, boot custom recovery - TWRP will suffice - format (not wipe!) userdata, push modified fstab.qcom with flag encryptable (or forceencrypt), reboot to bootloader, and boot device.

Not quite sure whether re-locking bootloader after changing fstab.qcom will cause a bootloop (did couple of lobotomies on that devices, but haven't tried this particular scenario).

Either way, if u try to re-lock bootloader &amp;&amp; that ends in bootloop, u can always flash a TOT file.

There r modified boot images for that device for same purpose, but with additional code - custom sepolicy for root.