kill struct filename.separate

just make const char iname[] the last member and compare name->name with
name->iname instead of checking name->separate

We need to make sure that out-of-line name doesn't end up allocated adjacent
to struct filename refering to it; fortunately, it's easy to achieve - just
allocate that struct filename with one byte in ->iname[], so that ->iname[0]
will be inside the same object and thus have an address different from that
of out-of-line name [spotted by Boqun Feng <boqun.feng@gmail.com>]

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

include/linux/fs.h

@@ -2144,7 +2144,7 @@ struct filename {
 	const __user char	*uptr;	/* original userland pointer */
 	struct audit_names	*aname;
 	int			refcnt;
-	bool			separate; /* should "name" be freed? */
+	const char		iname[];
 };
 
 extern long vfs_truncate(struct path *, loff_t);