drm/i915/gvt: fix user copy warning by whitelist workload rb_tail field

drm/i915/gvt: fix user copy warning by whitelist workload rb_tail field

This is to fix warning got as:

[ 6730.476938] ------------[ cut here ]------------
[ 6730.476979] Bad or missing usercopy whitelist? Kernel memory exposure attempt detected from SLAB object 'gvt-g_vgpu_workload' (offset 120, size 4)!
[ 6730.477021] WARNING: CPU: 2 PID: 441 at mm/usercopy.c:81 usercopy_warn+0x7e/0xa0
[ 6730.477042] Modules linked in: tun(E) bridge(E) stp(E) llc(E) kvmgt(E) x86_pkg_temp_thermal(E) vfio_mdev(E) intel_powerclamp(E) mdev(E) coretemp(E) vfio_iommu_type1(E) vfio(E) kvm_intel(E) kvm(E) hid_generic(E) irqbypass(E) crct10dif_pclmul(E) crc32_pclmul(E) usbhid(E) i915(E) crc32c_intel(E) hid(E) ghash_clmulni_intel(E) pcbc(E) aesni_intel(E) aes_x86_64(E) crypto_simd(E) cryptd(E) glue_helper(E) intel_cstate(E) idma64(E) evdev(E) virt_dma(E) iTCO_wdt(E) intel_uncore(E) intel_rapl_perf(E) intel_lpss_pci(E) sg(E) shpchp(E) mei_me(E) pcspkr(E) iTCO_vendor_support(E) intel_lpss(E) intel_pch_thermal(E) prime_numbers(E) mei(E) mfd_core(E) video(E) acpi_pad(E) button(E) binfmt_misc(E) ip_tables(E) x_tables(E) autofs4(E) ext4(E) crc16(E) mbcache(E) jbd2(E) fscrypto(E) sd_mod(E) e1000e(E) xhci_pci(E) sdhci_pci(E)
[ 6730.477244]  ptp(E) cqhci(E) xhci_hcd(E) pps_core(E) sdhci(E) mmc_core(E) i2c_i801(E) usbcore(E) thermal(E) fan(E)
[ 6730.477276] CPU: 2 PID: 441 Comm: gvt workload 0 Tainted: G            E    4.16.0-rc1-gvt-staging-0213+ #127
[ 6730.477303] Hardware name:  /NUC6i5SYB, BIOS SYSKLi35.86A.0039.2016.0316.1747 03/16/2016
[ 6730.477326] RIP: 0010:usercopy_warn+0x7e/0xa0
[ 6730.477340] RSP: 0018:ffffba6301223d18 EFLAGS: 00010286
[ 6730.477355] RAX: 0000000000000000 RBX: ffff8f41caae9838 RCX: 0000000000000006
[ 6730.477375] RDX: 0000000000000007 RSI: 0000000000000082 RDI: ffff8f41dad166f0
[ 6730.477395] RBP: 0000000000000004 R08: 0000000000000576 R09: 0000000000000000
[ 6730.477415] R10: ffffffffb1293fb2 R11: 00000000ffffffff R12: 0000000000000001
[ 6730.477447] R13: ffff8f41caae983c R14: ffff8f41caae9838 R15: 00007f183ca2b000
[ 6730.477467] FS:  0000000000000000(0000) GS:ffff8f41dad00000(0000) knlGS:0000000000000000
[ 6730.477489] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 6730.477506] CR2: 0000559462817291 CR3: 000000028b46c006 CR4: 00000000003626e0
[ 6730.477526] Call Trace:
[ 6730.477537]  __check_object_size+0x9c/0x1a0
[ 6730.477562]  __kvm_write_guest_page+0x45/0x90 [kvm]
[ 6730.477585]  kvm_write_guest+0x46/0x80 [kvm]
[ 6730.477599]  kvmgt_rw_gpa+0x9b/0xf0 [kvmgt]
[ 6730.477642]  workload_thread+0xa38/0x1040 [i915]
[ 6730.477659]  ? do_wait_intr_irq+0xc0/0xc0
[ 6730.477673]  ? finish_wait+0x80/0x80
[ 6730.477707]  ? clean_workloads+0x120/0x120 [i915]
[ 6730.477722]  kthread+0x111/0x130
[ 6730.477733]  ? _kthread_create_worker_on_cpu+0x60/0x60
[ 6730.477750]  ? exit_to_usermode_loop+0x6f/0xb0
[ 6730.477766]  ret_from_fork+0x35/0x40
[ 6730.477777] Code: 48 c7 c0 20 e3 25 b1 48 0f 44 c2 41 50 51 41 51 48 89 f9 49 89 f1 4d 89 d8 4c 89 d2 48 89 c6 48 c7 c7 78 e3 25 b1 e8 b2 bc e4 ff <0f> ff 48 83 c4 18 c3 48 c7 c6 09 d0 26 b1 49 89 f1 49 89 f3 eb
[ 6730.477849] ---[ end trace cae869c1c323e45a ]---

By whitelist guest page write from workload struct allocated from kmem cache.

Reviewed-by: Hang Yuan <hang.yuan@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
(cherry picked from commit 5627705406874df57fdfad3b4e0c9aedd3b007df)