mm/debug.c: fix __dump_page when mapping->host is not set

While debugging something, I added a dump_page() into do_swap_page(),
and I got the splat from below.  The issue happens when dereferencing
mapping->host in __dump_page():

  ...
  else if (mapping) {
	pr_warn("%ps ", mapping->a_ops);
	if (mapping->host->i_dentry.first) {
		struct dentry *dentry;
		dentry = container_of(mapping->host->i_dentry.first, struct dentry, d_u.d_alias);
		pr_warn("name:\"%pd\" ", dentry);
	}
  }
  ...

Swap address space does not contain an inode information, and so
mapping->host equals NULL.

Although the dump_page() call was added artificially into
do_swap_page(), I am not sure if we can hit this from any other path, so
it looks worth fixing it.  We can easily do that by checking
mapping->host first.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: 1c6fb1d ("mm: print more information about mapping in __dump_page")
Signed-off-by: Oscar Salvador <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/debug.c

@@ -79,7 +79,7 @@ void __dump_page(struct page *page, const char *reason)
 		pr_warn("ksm ");
 	else if (mapping) {
 		pr_warn("%ps ", mapping->a_ops);
-		if (mapping->host->i_dentry.first) {
+		if (mapping->host && mapping->host->i_dentry.first) {
 			struct dentry *dentry;
 			dentry = container_of(mapping->host->i_dentry.first, struct dentry, d_u.d_alias);
 			pr_warn("name:\"%pd\" ", dentry);