fs/locks: F_UNLCK extension for F_OFD_GETLK

Currently F_UNLCK with F_OFD_GETLK returns -EINVAL. This patch changes
it such that specifying F_UNLCK returns information only about OFD locks
that are owned by the given file description.

Cc: Jeff Layton <jlayton@kernel.org>
Cc: Chuck Lever <chuck.lever@oracle.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: Shuah Khan <shuah@kernel.org>
Cc: linux-kselftest@vger.kernel.org
Cc: linux-api@vger.kernel.org
Signed-off-by: Stas Sergeev <stsp2@yandex.ru>
Signed-off-by: Jeff Layton <jlayton@kernel.org>