s390/vmem: split pages when debug pagealloc is enabled

Since commit bb1520d5 ("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.

Fixes: bb1520d5

 ("s390/mm: start kernel with DAT enabled")
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>