Always call PHYS_TO_VM_PAGE() in is_managed(). Fast road for addresses
under first_page cannot be taken as this variable is connected only to
vm_page_array segment. There could be more segments in system like
the ones for various fictitious page ranges. These can be situated
under vm_page_array segment and so, they could be skipped before this
fix. However, as far as I know, there is no report associated with it.

While here, the return type of this function is changed from boolean_t
to bool type.

Reviewed by: kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D8502