new helper: audit_file()

... for situations when we don't have any candidate in pathnames - basically,
in descriptor-based syscalls.

[Folded the build fix for !CONFIG_AUDITSYSCALL configs from Chen Gang]

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/open.c b/fs/open.c
index de92c13..b1bf3d5 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -516,7 +516,7 @@
 	int err = -EBADF;
 
 	if (f.file) {
-		audit_inode(NULL, f.file->f_path.dentry, 0);
+		audit_file(f.file);
 		err = chmod_common(&f.file->f_path, mode);
 		fdput(f);
 	}
@@ -642,7 +642,7 @@
 	error = mnt_want_write_file(f.file);
 	if (error)
 		goto out_fput;
-	audit_inode(NULL, f.file->f_path.dentry, 0);
+	audit_file(f.file);
 	error = chown_common(&f.file->f_path, user, group);
 	mnt_drop_write_file(f.file);
 out_fput: