KVM: guest_memfd: Remove preparation tracking

guest_memfd currently uses the folio uptodate flag to track:

  1) whether or not a page has been cleared before initial usage
  2) whether or not the architecture hooks have been issued to put the
     page in a private state as defined by the architecture

In practice, (2) is only actually being tracked for SEV-SNP VMs, and
there do not seem to be any plans/reasons that would suggest this will
change in the future, so this additional tracking/complexity is not
really providing any general benefit to guest_memfd users.  On the other
hand, future plans around in-place conversion and hugepage support will
make the burden of tracking this information within guest_memfd even more
complex.

With in-place conversion and hugepage support, the plan is to use the
per-folio uptodate flag purely to track the initial clearing of folios,
whereas conversion operations could trigger multiple transitions between
'prepared' and 'unprepared' and thus need separate tracking.  Since
preparation generally happens during fault time, i.e. on the "read-side"
of any VM-wide locks that might protect state tracked by guest_memfd,
supporting concurrent handling of page faults would likely require more
complex locking schemes if the "preparedness" state were tracked by
guest_memfd, i.e. if it needs to be updated as part of handling the fault.

Instead of keeping this current/future complexity within guest_memfd for
what is essentially just SEV-SNP, just drop the tracking for (2) and have
the arch-specific preparation hooks get triggered unconditionally on
every fault so the arch-specific hooks can check the preparation state
directly and decide whether or not a folio still needs additional
preparation. In the case of SEV-SNP, the preparation state is already
checked again via the preparation hooks to avoid double-preparation, so
nothing extra needs to be done to update the handling of things there.

Reviewed-by: Vishal Annapurve <vannapurve@google.com>
Tested-by: Vishal Annapurve <vannapurve@google.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Tested-by: Kai Huang <kai.huang@intel.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>
Link: https://patch.msgid.link/20260108214622.1084057-4-michael.roth@amd.com
[sean: massage changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>

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