new helper: file_inode(file)

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

mm/filemap.c

@@ -1711,7 +1711,7 @@ EXPORT_SYMBOL(filemap_fault);
 int filemap_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 {
 	struct page *page = vmf->page;
-	struct inode *inode = vma->vm_file->f_path.dentry->d_inode;
+	struct inode *inode = file_inode(vma->vm_file);
 	int ret = VM_FAULT_LOCKED;
 
 	sb_start_pagefault(inode->i_sb);