homectl: make sure we sent the full 8 bytes as flags

homectl: make sure we sent the full 8 bytes as flags

Otherwise weird stuff happens on the other side:

[1217111.957263] testsuite-46.sh[61]: + homectl create test-user --disk-size=min --luks-discard=yes --image-path=/home/test-user.home --luks-pbkdf-type=pbkdf2 --luks-pbkdf-time-cost=1ms
[1217112.598219] homectl[66]: Operation on home test-user failed: Provided flags are unsupported (0ad2578000000000).

(taken from TEST-46-HOME run on armv7l)

Fixes issue mentioned in https://github.com/systemd/systemd/pull/31419#issuecomment-1955117397.