kdump: Write the correct address of mem_section into vmcoreinfo

Depending on configuration mem_section can now be an array or a pointer
to an array allocated dynamically. In most cases, we can continue to refer
to it as 'mem_section' regardless of what it is.

But there's one exception: '&mem_section' means "address of the array" if
mem_section is an array, but if mem_section is a pointer, it would mean
"address of the pointer".

We've stepped onto this in the kdump code: VMCOREINFO_SYMBOL(mem_section)
writes down the address of pointer into vmcoreinfo, not the array as we wanted,
breaking kdump.

Let's introduce VMCOREINFO_SYMBOL_ARRAY() that would handle the
situation correctly for both cases.

Mike Galbraith <efault@gmx.de>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Baoquan He <bhe@redhat.com>
Acked-by: Dave Young <dyoung@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: kexec@lists.infradead.org
Cc: linux-mm@kvack.org
Cc: stable@vger.kernel.org
Fixes: 83e3c487

 ("mm/sparsemem: Allocate mem_section at runtime for CONFIG_SPARSEMEM_EXTREME=y")
Link: http://lkml.kernel.org/r/20180112162532.35896-1-kirill.shutemov@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>