fs/proc: fix compile warning about variable 'vmcore_mmap_ops'

When build with !CONFIG_MMU, the variable 'vmcore_mmap_ops'
is defined but not used:

>> fs/proc/vmcore.c:458:42: warning: unused variable 'vmcore_mmap_ops'
     458 | static const struct vm_operations_struct vmcore_mmap_ops = {

Fix this by only defining it when CONFIG_MMU is enabled.

Link: https://lkml.kernel.org/r/20241101034803.9298-1-xiqi2@huawei.com
Fixes: 9cb2181 ("vmcore: introduce remap_oldmem_pfn_range()")
Signed-off-by: Qi Xi <xiqi2@huawei.com>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/lkml/202410301936.GcE8yUos-lkp@intel.com/
Cc: Baoquan He <bhe@redhat.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Wang ShaoBo <bobo.shaobowang@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Qi Xi

fs/proc/vmcore.c

@@ -457,10 +457,6 @@ static vm_fault_t mmap_vmcore_fault(struct vm_fault *vmf)
 #endif
 }
 
-static const struct vm_operations_struct vmcore_mmap_ops = {
-	.fault = mmap_vmcore_fault,
-};
-
 /**
  * vmcore_alloc_buf - allocate buffer in vmalloc memory
  * @size: size of buffer
@@ -488,6 +484,11 @@ static inline char *vmcore_alloc_buf(size_t size)
  * virtually contiguous user-space in ELF layout.
  */
 #ifdef CONFIG_MMU
+
+static const struct vm_operations_struct vmcore_mmap_ops = {
+	.fault = mmap_vmcore_fault,
+};
+
 /*
  * remap_oldmem_pfn_checked - do remap_oldmem_pfn_range replacing all pages
  * reported as not being ram with the zero page.