net: socket: don't set sk_uid to garbage value in ->setattr()

->setattr() was recently implemented for socket files to sync the socket
inode's uid to the new 'sk_uid' member of struct sock.  It does this by
copying over the ia_uid member of struct iattr.  However, ia_uid is
actually only valid when ATTR_UID is set in ia_valid, indicating that
the uid is being changed, e.g. by chown.  Other metadata operations such
as chmod or utimes leave ia_uid uninitialized.  Therefore, sk_uid could
be set to a "garbage" value from the stack.

Fix this by only copying the uid over when ATTR_UID is set.

Fixes: 86741ec2 ("net: core: Add a UID field to struct sock.")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Tested-by: Lorenzo Colitti <lorenzo@google.com>
Acked-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>