coredump: fix spam with zero VMA process

coredump: fix spam with zero VMA process

Nobody ever tried to self destruct by unmapping whole address space at
once:

	munmap((void *)0, (1ULL << 47) - 4096);

Doing this produces 2 warnings for zero-length vmalloc allocations:

  a.out[1353]: segfault at 7f80bcc4b757 ip 00007f80bcc4b757 sp 00007fff683939b8 error 14
  a.out: vmalloc: allocation failure: 0 bytes, mode:0xcc0(GFP_KERNEL), nodemask=(null)
	...
  a.out: vmalloc: allocation failure: 0 bytes, mode:0xcc0(GFP_KERNEL), nodemask=(null)
	...

Fix is to switch to kvmalloc().

Steps to reproduce:

	// vsyscall=none
	#include <sys/mman.h>
	#include <sys/resource.h>
	int main(void)
	{
		setrlimit(RLIMIT_CORE, &(struct rlimit){RLIM_INFINITY, RLIM_INFINITY});
		munmap((void *)0, (1ULL << 47) - 4096);
		return 0;
	}

Link: http://lkml.kernel.org/r/20180410180353.GA2515@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>