vmmon: fix return type of vm_operations_struct::fault handler

Commit 3d3539018d2c ("mm: create the new vm_fault_t type") in mainline
5.1-rc1 changed the definition of vm_fault_t type to unsigned to catch
vm_operations_struct::fault handlers which still have int as return value.
LinuxDriverFault() in vmmon module is one of those.

As handler return type was changed by commit 1c8f422059ae ("mm: change
return type to vm_fault_t") in 4.17-rc1, make LinuxDriverFault() always
return vm_fault_t and define vm_fault_t as int when building against
a pre-4.17 kernel.

vmmon-only/linux/driver.c

@@ -73,6 +73,9 @@ static Bool LinuxDriverCheckPadding(void);
 
 struct VMXLinuxState linuxState;
 
+#if LINUX_VERSION_CODE < KERNEL_VERSION(4, 17, 0)
+typedef int vm_fault_t;
+#endif
 
 /*
  *----------------------------------------------------------------------
@@ -97,9 +100,9 @@ long LinuxDriver_Ioctl(struct file *filp, u_int iocmd,
 
 static int LinuxDriver_Close(struct inode *inode, struct file *filp);
 #if LINUX_VERSION_CODE >= KERNEL_VERSION(4, 11, 0)
-static int LinuxDriverFault(struct vm_fault *fault);
+static vm_fault_t LinuxDriverFault(struct vm_fault *fault);
 #else
-static int LinuxDriverFault(struct vm_area_struct *vma, struct vm_fault *fault);
+static vm_fault_t LinuxDriverFault(struct vm_area_struct *vma, struct vm_fault *fault);
 #endif
 static int LinuxDriverMmap(struct file *filp, struct vm_area_struct *vma);
 
@@ -594,7 +597,7 @@ LinuxDriver_Close(struct inode *inode, // IN
  *-----------------------------------------------------------------------------
  */
 
-static int
+static vm_fault_t
 #if LINUX_VERSION_CODE >= KERNEL_VERSION(4, 11, 0)
 LinuxDriverFault(struct vm_fault *fault)     //IN/OUT
 #else

vmmon-only/linux/hostif.c

@@ -2184,7 +2184,7 @@ isVAReadable(VA r)  // IN:
    int ret;
 
    old_fs = get_fs();
-   set_fs(get_ds());
+   set_fs(KERNEL_DS);
    r = APICR_TO_ADDR(r, APICR_VERSION);
    ret = HostIF_CopyFromUser(&dummy, r, sizeof dummy);
    set_fs(old_fs);
@@ -2385,7 +2385,7 @@ HostIF_SemaphoreWait(VMDriver *vm,   // IN:
    }
 
    old_fs = get_fs();
-   set_fs(get_ds());
+   set_fs(KERNEL_DS);
 
    {
       struct poll_wqueues table;
@@ -2514,7 +2514,7 @@ HostIF_SemaphoreSignal(uint64 *args)  // IN:
    }
 
    old_fs = get_fs();
-   set_fs(get_ds());
+   set_fs(KERNEL_DS);
 
    /*
     * Always write sizeof(uint64) bytes. This works fine for eventfd and