s390/vmem: split pages when debug pagealloc is enabled

In the Linux kernel, the following vulnerability has been resolved:

s390/vmem: split pages when debug pagealloc is enabled

Since commit bb1520d581a3 ("s390/mm: start kernel with DAT enabled")
the kernel crashes early during boot when debug pagealloc is enabled:

mem auto-init: stack:off, heap alloc:off, heap free:off
addressing exception: 0005 ilc:2 [#1] SMP DEBUG_PAGEALLOC
Modules linked in:
CPU: 0 PID: 0 Comm: swapper Not tainted 6.5.0-rc3-09759-gc5666c912155 #630
[..]
Krnl Code: 00000000001325f6: ec5600248064 cgrj %r5,%r6,8,000000000013263e
00000000001325fc: eb880002000c srlg %r8,%r8,2
#0000000000132602: b2210051 ipte %r5,%r1,%r0,0
>0000000000132606: b90400d1 lgr %r13,%r1
000000000013260a: 41605008 la %r6,8(%r5)
000000000013260e: a7db1000 aghi %r13,4096
0000000000132612: b221006d ipte %r6,%r13,%r0,0
0000000000132616: e3d0d0000171 lay %r13,4096(%r13)

Call Trace:
__kernel_map_pages+0x14e/0x320
__free_pages_ok+0x23a/0x5a8)
free_low_memory_core_early+0x214/0x2c8
memblock_free_all+0x28/0x58
mem_init+0xb6/0x228
mm_core_init+0xb6/0x3b0
start_kernel+0x1d2/0x5a8
startup_continue+0x36/0x40
Kernel panic - not syncing: Fatal exception: panic_on_oops

This is caused by using large mappings on machines with EDAT1/EDAT2. Add
the code to split the mappings into 4k pages if debug pagealloc is enabled
by CONFIG_DEBUG_PAGEALLOC_ENABLE_DEFAULT or the debug_pagealloc kernel
command line option.