vm_phys; fix uncalled free_contig

Function vm_phys_free_contig does not always free memory properly when
the npages parameter is less than max block size. Change it so that it does.

Note that this function is not currently invoked, and this error was
not triggered in earlier versions of the code.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D42891