Fix breakage caused by r254466 in minidumpsys().

r254466 increased the KVA from 512GB to 2TB which requires 4 PDP pages as
opposed to a single one before the change. This broke minidumpsys() since
it assumed that the entire KVA could be addressed via a single PDP page.

Fix this by obtaining the address of the PDP page from the PML4 entry
associated with the KVA being dumped.

Reported by:	pho
Submitted by:	kib
Pointy hat to:	neel

(cherry picked from commit bea80a701c438014ad3a85212547369e82c77201)

Conflicts:
	sys/amd64/amd64/minidump_machdep.c