[PATCH] arm64: Get 'info->page_offset' from PT_LOAD segments to support KASLR boot cases

The existing methodology to obtain 'info->page_offset' from reading
_stext symbol (from kallsyms) doesn't work well in KASLR boot cases on
arm64 machines as the PAGE_OFFSET (or the virtual address which
indicates the start of the linear region) can be randomized as well
on basis of the kaslr-seed.

Since the value of PAGE_OFFSET inside the kernel is randomized in such
cases and there is no existing mechanism of conveying this value from
kernel-space to user-space, so we can use the method used by archs like
x86_64 to generate the 'info->page_offset' value from the PT_LOAD
segments by subtracting the phy_addr from virt_addr of a PT_LOAD
segment.

This approach works fine both with KASLR and non-KASLR boot cases.

I tested this on my qualcomm-amberwing board. Here are some logs from
the KASLR boot cases:

- Verify that the EFI firmware supports 'kaslr-seed':

	chosen {
		kaslr-seed = <0x0 0x0>;
		<..snip..>
	};

- Verify that '--mem-usage' works well after this fix as well (I used
  kernel 4.18.0-rc4+ for my checks):

The kernel version is not supported.
The makedumpfile operation may be incomplete.

TYPE		PAGES			EXCLUDABLE	DESCRIPTION
----------------------------------------------------------------------
ZERO		4396            	yes		Pages filled with zero
NON_PRI_CACHE	27859           	yes		Cache pages without private flag
PRI_CACHE	18490           	yes		Cache pages with private flag
USER		2728            	yes		User process pages
FREE		1465848         	yes		Free pages
KERN_DATA	18537           	no		Dumpable kernel data

page size:		65536
Total pages on system:	1537858
Total size on system:	100785061888     Byte

Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>
Signed-off-by: Kazuhito Hagio <k-hagio@ab.jp.nec.com>