vfs: embed struct filename inside of names_cache allocation if possible

In the common case where a name is much smaller than PATH_MAX, an extra
allocation for struct filename is unnecessary. Before allocating a
separate one, try to embed the struct filename inside the buffer first. If
it turns out that that's not long enough, then fall back to allocating a
separate struct filename and redoing the copy.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/fs.h

@@ -2201,6 +2201,7 @@ struct filename {
 	const char		*name;	/* pointer to actual string */
 	const __user char	*uptr;	/* original userland pointer */
 	struct audit_names	*aname;
+	bool			separate; /* should "name" be freed? */
 };
 
 extern int do_truncate(struct dentry *, loff_t start, unsigned int time_attrs,