vm_phys: convert error back to warning

Move an assignment back to where it was before, to turn the
defined-but-not-used error back into a set-but-not-used warning.

Fixes: 01e115ab83a4 vm_phys: #include vm_extern
(cherry picked from commit e6930b1c5f8f9307f72eda1b5b3c504dc009cbc6)