KVM: fix rcu usage warning in kvm_arch_vcpu_ioctl_set_sregs()
Fix:
[ 1001.499596] ===================================================
[ 1001.499599] [ INFO: suspicious rcu_dereference_check() usage. ]
[ 1001.499601] ---------------------------------------------------
[ 1001.499604] include/linux/kvm_host.h:301 invoked rcu_dereference_check() without protection!
......
[ 1001.499636] Pid: 6035, comm: qemu-system-x86 Not tainted 2.6.37-rc6+ #62
[ 1001.499638] Call Trace:
[ 1001.499644] [] lockdep_rcu_dereference+0x9d/0xa5
[ 1001.499653] [] gfn_to_memslot+0x8d/0xc8 [kvm]
[ 1001.499661] [] gfn_to_hva+0x16/0x3f [kvm]
[ 1001.499669] [] kvm_read_guest_page+0x1e/0x5e [kvm]
[ 1001.499681] [] kvm_read_guest_page_mmu+0x53/0x5e [kvm]
[ 1001.499699] [] load_pdptrs+0x3f/0x9c [kvm]
[ 1001.499705] [] ? vmx_set_cr0+0x507/0x517 [kvm_intel]
[ 1001.499717] [] kvm_arch_vcpu_ioctl_set_sregs+0x1f3/0x3c0 [kvm]
[ 1001.499727] [] kvm_vcpu_ioctl+0x6a5/0xbc5 [kvm]
Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>