drm/i915/gvt: request srcu_read_lock before checking if one gfn is valid

drm/i915/gvt: request srcu_read_lock before checking if one gfn is valid

Fix the suspicious RCU usage issue in intel_vgpu_emulate_mmio_write.
Here need to request the srcu read lock of kvm->srcu before doing
gfn_to_memslot(). The detailed log is as below:
[  218.710688] =============================
[  218.710690] WARNING: suspicious RCU usage
[  218.710693] 4.14.15-dd+ #314 Tainted: G     U
[  218.710695] -----------------------------
[  218.710697] ./include/linux/kvm_host.h:575 suspicious rcu_dereference_check() usage!
[  218.710699]
               other info that might help us debug this:

[  218.710702]
               rcu_scheduler_active = 2, debug_locks = 1
[  218.710704] 1 lock held by qemu-system-x86/2144:
[  218.710706]  #0:  (&gvt->lock){+.+.}, at: [<ffffffff816a1eea>] intel_vgpu_emulate_mmio_write+0x5a/0x2d0
[  218.710721]
               stack backtrace:
[  218.710724] CPU: 0 PID: 2144 Comm: qemu-system-x86 Tainted: G     U 4.14.15-dd+ #314
[  218.710727] Hardware name: Dell Inc. OptiPlex 7040/0Y7WYT, BIOS 1.1.1 10/07/2015
[  218.710729] Call Trace:
[  218.710734]  dump_stack+0x7c/0xb3
[  218.710739]  gfn_to_memslot+0x15f/0x170
[  218.710743]  kvm_is_visible_gfn+0xa/0x30
[  218.710746]  intel_vgpu_emulate_gtt_mmio_write+0x267/0x3c0
[  218.710751]  ? __mutex_unlock_slowpath+0x3b/0x260
[  218.710754]  intel_vgpu_emulate_mmio_write+0x182/0x2d0
[  218.710759]  intel_vgpu_rw+0xba/0x170 [kvmgt]
[  218.710763]  intel_vgpu_write+0x14d/0x1a0 [kvmgt]
[  218.710767]  __vfs_write+0x23/0x130
[  218.710770]  vfs_write+0xb0/0x1b0
[  218.710774]  SyS_pwrite64+0x73/0x90
[  218.710777]  entry_SYSCALL_64_fastpath+0x25/0x9c
[  218.710780] RIP: 0033:0x7f33e8a91da3
[  218.710783] RSP: 002b:00007f33dddc8700 EFLAGS: 00000293

v2: add 'Fixes' tag, refine log format.(Zhenyu)
Fixes: cc753fbe1ac4 ("drm/i915/gvt: validate gfn before set shadow page")
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Weinan Li <weinan.z.li@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>