kdump, vmcoreinfo: report actual value of phys_base

Currently, VMCOREINFO note information reports the virtual address of
phys_base that is assigned to symbol phys_base.  But this doesn't make
sense because to refer to phys_base, it's necessary to get the value of
phys_base itself we are now about to refer to.

Userland tools related to kdump such as makedumpfile and crash utility so
far have made some efforts to calculate phys_base on crash dump formats
generated by mechanisms running outside Linux kernel, such as virtual
machine hypervisor such as qemu dump, which ordinary users use via virsh
dump, or ones implemented on vendor specific firmware.

That is, find a kernel data whose virtual and physical addresses are
available via its note information and calculate phys_base from it.
However, such data structure is not the one prepared for phys_base
purpose.  There's no guarantee that other crash dump mechanisms include
such information that can be used to calculate phys_base similarly.

To get VMCOREINFO in vmcore, it's easy to use strings and grep commands
like this; VMCOREINFO consists of simple string:

$ strings vmcore-3.10.0-121.el7.x86_64 | grep -E ".*VMCOREINFO.*" -A 100
VMCOREINFO
OSRELEASE=3.10.0-121.el7.x86_64
PAGESIZE=4096
...

This is also useful to get value of phys_base in kdump 2nd kernel
contained in vmcore using the above-mentioned external crash dump
mechanism; kdump 2nd kernel is an inherently relocated kernel.

This commit doesn't remove VMCOREINFO_SYMBOL(phys_base) line because
makedumpfile refers to it and if removing it, old versions makedumpfile
doesn't work well.

Signed-off-by: HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Atsushi Kumagai <kumagai-atsushi@mxc.nes.nec.co.jp>
Cc: Dave Anderson <anderson@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>