powerpc: insert System RAM resource to prevent crashkernel conflict

The next patch in the series with title "powerpc/crash: use generic
crashkernel reservation" enables powerpc to use generic crashkernel
reservation instead of custom implementation.  This leads to exporting of
`Crash Kernel` memory to iomem_resource (/proc/iomem) via
insert_crashkernel_resources():kernel/crash_reserve.c or at another place
in the same file if HAVE_ARCH_ADD_CRASH_RES_TO_IOMEM_EARLY is set.

The add_system_ram_resources():arch/powerpc/mm/mem.c adds `System RAM` to
iomem_resource using request_resource().  This creates a conflict with
`Crash Kernel`, which is added by the generic crashkernel reservation
code.  As a result, the kernel ultimately fails to add `System RAM` to
iomem_resource.  Consequently, it does not appear in /proc/iomem.

There are multiple approches tried to avoid this:

1. Don't add Crash Kernel to iomem_resource:
    - This has two issues.
      First, it requires adding an architecture-specific hook in the
      generic code. There are already two code paths to choose when to
      add `Crash Kernel` to iomem_resource. This adds one more code path
      to skip it.

      Second, what if `Crash Kernel` is required in /proc/iomem in the
      future? Many architectures do export it.

2. Don't add `System RAM` to iomem_resource by reverting commit
   c40dd2f766 ("powerpc: Add System RAM to /proc/iomem"):
    - It's not ideal to export `System RAM` via /proc/iomem, but since
      it already done ealier and userspace tools like kdump and
      kdump-utils rely on `System RAM` from /proc/iomem, removing it
      will break userspace.

3. Add Crash Kernel along with System RAM to /proc/iomem:

This patch takes the third approach by updating add_system_ram_resources()
to use insert_resource() instead of the request_resource() API to add the
`System RAM` resource to iomem_resource.  insert_resource() allows
inserting resources even if they overlap with existing ones.  Since `Crash
Kernel` and `System RAM` resources are added to iomem_resource early in
the boot, any other conflict is not expected.

With the changes introduced here and in the next patch, "powerpc/crash:
use generic crashkernel reservation," /proc/iomem now exports `System RAM`
and `Crash Kernel` as shown below:

$ cat /proc/iomem
00000000-3ffffffff : System RAM
  10000000-4fffffff : Crash kernel

The kdump script is capable of handling `System RAM` and `Crash Kernel` in
the above format.  The same format is used in other architectures.

Link: https://lkml.kernel.org/r/20250131113830.925179-7-sourabhjain@linux.ibm.com
Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Cc: Baoquan he <bhe@redhat.com>
Cc: Hari Bathini <hbathini@linux.ibm.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Mahesh Salgaonkar <mahesh@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

arch/powerpc/mm/mem.c

@@ -376,7 +376,7 @@ static int __init add_system_ram_resources(void)
 			 */
 			res->end = end - 1;
 			res->flags = IORESOURCE_SYSTEM_RAM | IORESOURCE_BUSY;
-			WARN_ON(request_resource(&iomem_resource, res) < 0);
+			WARN_ON(insert_resource(&iomem_resource, res) < 0);
 		}
 	}