KVM: Don't BUG() the kernel if xa_insert() fails with -EBUSY

KVM: Don't BUG() the kernel if xa_insert() fails with -EBUSY

WARN once instead of triggering a BUG if xa_insert() fails because it
encountered an existing entry.  While KVM guarantees there should be no
existing entry, there's no reason to BUG the kernel, as KVM needs to
gracefully handle failure anyways.

Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20241009150455.1057573-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>