KVM: Dynamically allocate "new" memslots from the get-go

Allocate the "new" memslot for !DELETE memslot updates straight away
instead of filling an intermediate on-stack object and forcing
kvm_set_memslot() to juggle the allocation and do weird things like reuse
the old memslot object in MOVE.

In the MOVE case, this results in an "extra" memslot allocation due to
allocating both the "new" slot and the "invalid" slot, but that's a
temporary and not-huge allocation, and MOVE is a relatively rare memslot
operation.

Regarding MOVE, drop the open-coded management of the gfn tree with a
call to kvm_replace_memslot(), which already handles the case where
new->base_gfn != old->base_gfn.  This is made possible by virtue of not
having to copy the "new" memslot data after erasing the old memslot from
the gfn tree.  Using kvm_replace_memslot(), and more specifically not
reusing the old memslot, means the MOVE case now does hva tree and hash
list updates, but that's a small price to pay for simplifying the code
and making MOVE align with all the other flavors of updates.  The "extra"
updates are firmly in the noise from a performance perspective, e.g. the
"move (in)active area" selfttests show a (very, very) slight improvement.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Message-Id: <f0d8c72727aa825cf682bd4e3da4b3fa68215dd4.1638817641.git.maciej.szmigiero@oracle.com>

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