Yes, it is available. Earlier the feature was not accessible on the FB App. Instead, it was only possible to do so by accessing the web version. But, now, post the new FB update, and you can lock your Profile. Follow these steps to lock the Profile:
Open FB app on iOS device.
Once logged in, tap on the menu icon on the bottom corner
Tap on Settings at the right top corner
This will open the Profile Settings
Once you tap on Profile, you will see Privacy settings
Under that, tap on Profile locking
Tap on lock your Profile.
Only those who have been granted full access will be able to view the Profile.