fs/proc/vmcore.c: hide vmcoredd_mmap_dumps() for nommu builds

fs/proc/vmcore.c: hide vmcoredd_mmap_dumps() for nommu builds

Without CONFIG_MMU, we get a build warning:

  fs/proc/vmcore.c:228:12: error: 'vmcoredd_mmap_dumps' defined but not used [-Werror=unused-function]
   static int vmcoredd_mmap_dumps(struct vm_area_struct *vma, unsigned long dst,

The function is only referenced from an #ifdef'ed caller, so
this uses the same #ifdef around it.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: 7efe48df8a3d ("vmcore: append device dumps to vmcore as elf notes")
Signed-off-by: Arnd Bergmann <[email protected]>
Cc: Ganesh Goudar <[email protected]>
Cc: "David S. Miller" <[email protected]>
Cc: Rahul Lakkireddy <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>