ARM: kexec: avoid allocating crashkernel region outside lowmem

ARM: kexec: avoid allocating crashkernel region outside lowmem

Allocating the crashkernel region outside lowmem causes the kernel to
oops while trying to kexec into the new kernel:

Loading crashdump kernel...
Unable to handle kernel NULL pointer dereference at virtual address 00000000
pgd = edd70000
[00000000] *pgd=de19e835
Internal error: Oops: 817 [#2] SMP ARM
Modules linked in: ...
CPU: 0 PID: 689 Comm: sh Not tainted 4.12.0-rc3-next-20170601-04015-gc3a5a20
Hardware name: Generic DRA74X (Flattened Device Tree)
task: edb32f00 task.stack: edf18000
PC is at memcpy+0x50/0x330
LR is at 0xe3c34001
pc : [<c04baf30>]    lr : [<e3c34001>]    psr: 800c0193
sp : edf19c2c  ip : 0a000001  fp : c0553170
r10: c055316e  r9 : 00000001  r8 : e3130001
r7 : e4903004  r6 : 0a000014  r5 : e3500000  r4 : e59f106c
r3 : e59f0074  r2 : ffffffe8  r1 : c010fb88  r0 : 00000000
Flags: Nzcv  IRQs off  FIQs on  Mode SVC_32  ISA ARM  Segment none
Control: 10c5387d  Table: add7006a  DAC: 00000051
Process sh (pid: 689, stack limit = 0xedf18218)
Stack: (0xedf19c2c to 0xedf1a000)
...
[<c04baf30>] (memcpy) from [<c010fae0>] (machine_kexec+0xa8/0x12c)
[<c010fae0>] (machine_kexec) from [<c01e4104>] (__crash_kexec+0x5c/0x98)
[<c01e4104>] (__crash_kexec) from [<c01e419c>] (crash_kexec+0x5c/0x68)
[<c01e419c>] (crash_kexec) from [<c010c5c0>] (die+0x228/0x490)
[<c010c5c0>] (die) from [<c011e520>] (__do_kernel_fault.part.0+0x54/0x1e4)
[<c011e520>] (__do_kernel_fault.part.0) from [<c082412c>] (do_page_fault+0x1e8/0x400)
[<c082412c>] (do_page_fault) from [<c010135c>] (do_DataAbort+0x38/0xb8)
[<c010135c>] (do_DataAbort) from [<c0823584>] (__dabt_svc+0x64/0xa0)

This is caused by image->control_code_page being a highmem page, so
page_address(image->control_code_page) returns NULL.  In any case, we
don't want the control page to be a highmem page.

We already limit the crash kernel region to the top of 32-bit physical
memory space.  Also limit it to the top of lowmem in physical space.

Reported-by: Keerthy <j-keerthy@ti.com>
Tested-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>