Use explicit int conversion in bootmem.c

[quangIO]

Without the explicit cast, I encountered the following issue when using gcc 14 on Arch Linux:

❯ make -C tools/lkl
scripts/Makefile.asm-generic:25: redundant generic-y found in arch/lkl/include/uapi/asm/Kbuild: siginfo.h
scripts/Makefile.asm-generic:25: redundant generic-y found in arch/lkl/include/asm/Kbuild: atomic.h barrier.h bitops.h bug.h bugs.h cacheflush.h checksum.h compat.h current.h delay.h device.h div64.h emergency-restart.h exec.h ftrace.h futex.h hardirq.h hw_irq.h irq_regs.h irq_work.h kdebug.h kprobes.h linkage.h local.h local64.h mmiowb.h mmu.h module.h percpu.h pgalloc.h preempt.h sections.h serial.h simd.h switch_to.h timex.h tlbflush.h topology.h trace_clock.h uaccess.h unaligned.h vga.h word-at-a-time.h
  CALL    scripts/checksyscalls.sh
<stdin>:1519:2: warning: #warning syscall clone3 not implemented [-Wcpp]
  CC      arch/lkl/mm/bootmem.o
arch/lkl/mm/bootmem.c: In function ‘bootmem_init’:
arch/lkl/mm/bootmem.c:39:35: error: passing argument 1 of ‘virt_to_pfn’ makes pointer from integer without a cast [-Wint-conversion]
   39 |         max_low_pfn = virt_to_pfn(memory_end);
      |                                   ^~~~~~~~~~
      |                                   |
      |                                   long unsigned int
In file included from ./arch/lkl/include/asm/page.h:11,
                 from ./include/linux/mm_types_task.h:16,
                 from ./include/linux/mm_types.h:5,
                 from ./include/linux/mmzone.h:22,
                 from ./include/linux/gfp.h:7,
                 from ./include/linux/mm.h:7,
                 from ./include/linux/memblock.h:12,
                 from arch/lkl/mm/bootmem.c:2:
./include/asm-generic/page.h:77:53: note: expected ‘const void *’ but argument is of type ‘long unsigned int’
   77 | static inline unsigned long virt_to_pfn(const void *kaddr)
      |                                         ~~~~~~~~~~~~^~~~~
make[3]: *** [scripts/Makefile.build:243: arch/lkl/mm/bootmem.o] Error 1
make[2]: *** [scripts/Makefile.build:480: arch/lkl/mm] Error 2
make[1]: *** [Makefile:1917: .] Error 2
make: *** [Makefile:84: /home/lumi/dev/linux/tools/lkl/lib/lkl.o] Error 2