smaps: fix defined but not used smaps_shmem_walk_ops

When !CONFIG_SHMEM smaps_shmem_walk_ops is defined but not used,
triggering a compiler warning. To avoid the warning remove the #ifdef
around the usage. This has no effect because shmem_mapping() is a stub
returning false when !CONFIG_SHMEM so the code will be compiled out,
however we now need to also provide a stub for shmem_swap_usage().

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 7b86ac3371b7 ("pagewalk: separate function pointers from iterator data")
Signed-off-by: Steven Price <[e-mail]>
Reported-by: kernel test robot <[e-mail]>
Link: https://lore.kernel.org/oe-kbuild-all/[e-mail]/
Reviewed-by: Jason Gunthorpe <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Cc: Alexey Dobriyan <[e-mail]>
Cc: Thomas Hellström <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

fs/proc/task_mmu.c

···
 	if (start >= vma->vm_end)
 		return;
 
-#ifdef CONFIG_SHMEM
 	if (vma->vm_file && shmem_mapping(vma->vm_file->f_mapping)) {
 		/*
 		 * For shared or readonly shmem mappings we know that all
···
 			ops = &smaps_shmem_walk_ops;
 		}
 	}
-#endif
+
 	/* mmap_lock is held in m_start */
 	if (!start)
 		walk_page_vma(vma, ops, mss);

include/linux/shmem_fs.h

···
 
 extern bool shmem_is_huge(struct inode *inode, pgoff_t index, bool shmem_huge_force,
 			  struct mm_struct *mm, unsigned long vm_flags);
+#ifdef CONFIG_SHMEM
 extern unsigned long shmem_swap_usage(struct vm_area_struct *vma);
+#else
+static inline unsigned long shmem_swap_usage(struct vm_area_struct *vma)
+{
+	return 0;
+}
+#endif
 extern unsigned long shmem_partial_swap_usage(struct address_space *mapping,
 						pgoff_t start, pgoff_t end);