sadump: Fix a problem of PTI enabled kernel

This patch fixes a problme that a dumpfile of sadump cannot be handled by
makedumpfile when Page Table Isolation(PTI) is enabled.

When PTI is enabled, bit 12 of CR3 register is used to split user space and
kernel space. Also bit 11:0 is used for Process Context IDentifiers(PCID).  To
open a dump file of sadump, a value of CR3 is used to calculate KASLR offset and
phys_base, therefore this patch fixes to mask CR3 register value collectly for
PTI enabled kernel.

Signed-off-by: Takao Indoh <indou.takao@jp.fujitsu.com>