misc: pci_endpoint_test: Fix pci_endpoint_test_{copy,write,read}() panic

In the Linux kernel, the following vulnerability has been resolved:

misc: pci_endpoint_test: Fix pci_endpoint_test_{copy,write,read}() panic

The dma_map_single() doesn't permit zero length mapping. It causes a follow
panic.

A panic was reported on arm64:

[ 60.137988] ------------[ cut here ]------------
[ 60.142630] kernel BUG at kernel/dma/swiotlb.c:624!
[ 60.147508] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
[ 60.152992] Modules linked in: dw_hdmi_cec crct10dif_ce simple_bridge rcar_fdp1 vsp1 rcar_vin videobuf2_vmalloc rcar_csi2 v4l
2_mem2mem videobuf2_dma_contig videobuf2_memops pci_endpoint_test videobuf2_v4l2 videobuf2_common rcar_fcp v4l2_fwnode v4l2_asyn
c videodev mc gpio_bd9571mwv max9611 pwm_rcar ccree at24 authenc libdes phy_rcar_gen3_usb3 usb_dmac display_connector pwm_bl
[ 60.186252] CPU: 0 PID: 508 Comm: pcitest Not tainted 6.0.0-rc1rpci-dev+ #237
[ 60.193387] Hardware name: Renesas Salvator-X 2nd version board based on r8a77951 (DT)
[ 60.201302] pstate: 00000005 (nzcv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 60.208263] pc : swiotlb_tbl_map_single+0x2c0/0x590
[ 60.213149] lr : swiotlb_map+0x88/0x1f0
[ 60.216982] sp : ffff80000a883bc0
[ 60.220292] x29: ffff80000a883bc0 x28: 0000000000000000 x27: 0000000000000000
[ 60.227430] x26: 0000000000000000 x25: ffff0004c0da20d0 x24: ffff80000a1f77c0
[ 60.234567] x23: 0000000000000002 x22: 0001000040000010 x21: 000000007a000000
[ 60.241703] x20: 0000000000200000 x19: 0000000000000000 x18: 0000000000000000
[ 60.248840] x17: 0000000000000000 x16: 0000000000000000 x15: ffff0006ff7b9180
[ 60.255977] x14: ffff0006ff7b9180 x13: 0000000000000000 x12: 0000000000000000
[ 60.263113] x11: 0000000000000000 x10: 0000000000000000 x9 : 0000000000000000
[ 60.270249] x8 : 0001000000000010 x7 : ffff0004c6754b20 x6 : 0000000000000000
[ 60.277385] x5 : ffff0004c0da2090 x4 : 0000000000000000 x3 : 0000000000000001
[ 60.284521] x2 : 0000000040000000 x1 : 0000000000000000 x0 : 0000000040000010
[ 60.291658] Call trace:
[ 60.294100] swiotlb_tbl_map_single+0x2c0/0x590
[ 60.298629] swiotlb_map+0x88/0x1f0
[ 60.302115] dma_map_page_attrs+0x188/0x230
[ 60.306299] pci_endpoint_test_ioctl+0x5e4/0xd90 [pci_endpoint_test]
[ 60.312660] __arm64_sys_ioctl+0xa8/0xf0
[ 60.316583] invoke_syscall+0x44/0x108
[ 60.320334] el0_svc_common.constprop.0+0xcc/0xf0
[ 60.325038] do_el0_svc+0x2c/0xb8
[ 60.328351] el0_svc+0x2c/0x88
[ 60.331406] el0t_64_sync_handler+0xb8/0xc0
[ 60.335587] el0t_64_sync+0x18c/0x190
[ 60.339251] Code: 52800013 d2e00414 35fff45c d503201f (d4210000)
[ 60.345344] ---[ end trace 0000000000000000 ]---

To fix it, this patch adds a checking the payload length if it is zero.