libceph: factor out encrypt_authorizer()

commit 149cac4a50b0b4081b38b2f38de6ef71c27eaa85 upstream.

Will be used for encrypting both the initial and updated authorizers.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Sage Weil <sage@redhat.com>
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Sasha Levin <sashal@kernel.org>