audit: remove unnecessary NULL ptr checks from do_path_lookup

As best I can tell, whenever retval == 0, nd->path.dentry and nd->inode
are also non-NULL. Eliminate those checks and the superfluous
audit_context check.

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