mm/memory_hotplug: restrict CONFIG_MEMORY_HOTPLUG to 64 bit

32 bit support is broken in various ways: for example, we can online
memory that should actually go to ZONE_HIGHMEM to ZONE_MOVABLE or in
some cases even to one of the other kernel zones.

We marked it BROKEN in commit b59d02ed08 ("mm/memory_hotplug: disable
the functionality for 32b") almost one year ago.  According to that
commit it might be broken at least since 2017.  Further, there is hardly
a sane use case nowadays.

Let's just depend completely on 64bit, dropping the "BROKEN" dependency
to make clear that we are not going to support it again.  Next, we'll
remove some HIGHMEM leftovers from memory hotplug code to clean up.

Link: https://lkml.kernel.org/r/20210929143600.49379-4-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Cc: Alex Shi <alexs@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/Kconfig

@@ -125,7 +125,7 @@ config MEMORY_HOTPLUG
 	select MEMORY_ISOLATION
 	depends on SPARSEMEM
 	depends on ARCH_ENABLE_MEMORY_HOTPLUG
-	depends on 64BIT || BROKEN
+	depends on 64BIT
 	select NUMA_KEEP_MEMINFO if NUMA
 
 config MEMORY_HOTPLUG_DEFAULT_ONLINE