KVM: fix rcu warning on VM_CREATE errors

commit 3898da94 ("KVM: avoid using rcu_dereference_protected") can
trigger the following lockdep/rcu splat if the VM_CREATE ioctl fails,
for example if kvm_arch_init_vm fails:

WARNING: suspicious RCU usage
4.13.0+ #105 Not tainted
-----------------------------
./include/linux/kvm_host.h:481 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
no locks held by qemu-system-s39/79.
stack backtrace:
CPU: 0 PID: 79 Comm: qemu-system-s39 Not tainted 4.13.0+ #105
Hardware name: IBM 2964 NC9 704 (KVM/Linux)
Call Trace:
([<00000000001140b2>] show_stack+0xea/0xf0)
 [<00000000008a68a4>] dump_stack+0x94/0xd8
 [<0000000000134c12>] kvm_dev_ioctl+0x372/0x7a0
 [<000000000038f940>] do_vfs_ioctl+0xa8/0x6c8
 [<0000000000390004>] SyS_ioctl+0xa4/0xb8
 [<00000000008c7a8c>] system_call+0xc4/0x27c
no locks held by qemu-system-s39/79.

We have to reset the just created users_count back to 0 to
tell the check to not trigger.

Reported-by: Stefan Haberland <sth@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Fixes: 3898da94 ("KVM: avoid using rcu_dereference_protected")
Cc: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>