KVM: Add irqfd to KVM's list via the vfs_poll() callback

KVM: Add irqfd to KVM's list via the vfs_poll() callback

Add the irqfd structure to KVM's list of irqfds in kvm_irqfd_register(),
i.e. via the vfs_poll() callback.  This will allow taking irqfds.lock
across the entire registration sequence (add to waitqueue, add to list),
and more importantly will allow inserting into KVM's list if and only if
adding to the waitqueue succeeds (spoiler alert), without needing to
juggle return codes in weird ways.

Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250522235223.3178519-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>