snapuserd: Restrict where reads/writes to dm_user_header happen.

Only write to dm_user_header in the functions which explicitly need to
marshal it. This avoids leakage of dm-user specifics into core logic.

This also simplifies the existing control flow by allowing us to set an
error anywhere, or nowhere, as any "return false" from ProcessIORequest
will automatically set an error header.

Bug: 288273605
Test: snapuserd_test
Change-Id: I85f67208197d7ecc49e348ab3013827a38e84761