vm_phys: Change the return type of vm_phys_unfree_page() to bool

This is in keeping with the trend of removing uses of boolean_t, and the
sole caller was implicitly converting it to a "bool".

No functional change intended.

Reviewed by: dougm, alc, imp, kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D40401