KVM: Disallow binding multiple irqfds to an eventfd with a priority waiter

Disallow binding an irqfd to an eventfd that already has a priority waiter,
i.e. to an eventfd that already has an attached irqfd.  KVM always
operates in exclusive mode for EPOLL_IN (unconditionally returns '1'),
i.e. only the first waiter will be notified.

KVM already disallows binding multiple irqfds to an eventfd in a single
VM, but doesn't guard against multiple VMs binding to an eventfd.  Adding
the extra protection reduces the pain of a userspace VMM bug, e.g. if
userspace fails to de-assign before re-assigning when transferring state
for intra-host migration, then the migration will explicitly fail as
opposed to dropping IRQs on the destination VM.

Temporarily keep KVM's manual check on irqfds.items, but add a WARN, e.g.
to allow sanity checking the waitqueue enforcement.

Cc: Oliver Upton <[email protected]>
Cc: David Matlack <[email protected]>
Tested-by: K Prateek Nayak <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>

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