KEYS: store reference to machine keyring

Expose the .machine keyring created in integrity code by adding
a reference.  Store a reference to the machine keyring in
system keyring code. The system keyring code needs this to complete
the keyring link to the machine keyring.

Signed-off-by: Eric Snowberg <eric.snowberg@oracle.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Tested-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>