By the time that vm_reserv_init() runs, vm_phys_segs[] is initialized.  Use
it instead of phys_avail[].

Discussed with:	Svatopluk Kraus


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274867 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f