vmf_insert_pfn got added as a wrapper around vm_insert_pfn in 4.17
1c8f422059ae5da ("mm: change return type to vm_fault_t") and totally
replaced the later in 4.20 ae2b01f37044c ("mm: remove vm_insert_pfn()")
Compare with 4.18 here specifically to avoid troubles when rhel
backports this change later, and avoid adding a rhel version check down
the road.
Change-Id: Ibf108e2fb6f1199f89cde6a7973f4eb55447260b