DellSMM: gate on the Dell signature and run SMM calls on CPU 0

The module loaded on any x64 machine and forwarded caller controlled SMM
requests without ever establishing that the firmware is Dell's, so on
other systems those requests went into unrelated firmware SMI handlers.
It also never pinned itself, although the interface is only defined on
CPU 0 and can hang or return unusable results elsewhere.

Route every SMM entry through a helper that sets affinity to CPU 0 and
restores it afterwards, both checked, and refuse to load unless the
firmware answers one of the two signature requests with DIAG/DELL, which
is how the i8k / dell-smm-hwmon interface identifies itself.