hyperv/hn: Change global tunable prefix to hw.hn

[sepherosa_gmail.com]

Summary

And use SYSCTL+CTLFLAG_RDTUN for them.

Suggested by: adrian

[honzhan_microsoft.com]

LGTM

head/sys/dev/hyperv/netvsc/hv_netvsc_drv_freebsd.c

 #if VM_NRESERVLEVEL > 0
 #define KVA_QUANTUM_SHIFT       (VM_LEVEL_0_ORDER + PAGE_SHIFT)
 #else
 /* On non-superpage architectures we want large import sizes. */
 #define KVA_QUANTUM_SHIFT       (8 + PAGE_SHIFT)
 #endif
 #define KVA_QUANTUM             (1 << KVA_QUANTUM_SHIFT)
 
+extern void     uma_startup2(void);
+
 /*
  *      kva_alloc:
  *
  *      Allocate a virtual address range with no underlying object and
  *      no initial mapping to physical memory.  Any mapping from this
  *      range to physical memory must be explicitly created prior to
  *      its use, typically with pmap_qenter().  Any attempt to create
  *      a mapping on demand through vm_fault() will result in a panic. 

 #if VM_NRESERVLEVEL > 0
                 vm_dom[domain].vmd_kernel_rwx_arena = vmem_create(
                     "kernel rwx arena domain", 0, 0, PAGE_SIZE, 0, M_WAITOK);
                 vmem_set_import(vm_dom[domain].vmd_kernel_rwx_arena,
                     kva_import_domain, (vmem_release_t *)vmem_xfree,
                     kernel_arena, KVA_QUANTUM);
 #endif
         }
+
+        /*
+         * This must be the very first call so that the virtual address
+         * space used for early allocations is properly marked used in
+         * the map.
+         */
+        uma_startup2();
 }
 
 /*
  *      kmem_bootstrap_free:
  *
  *      Free pages backing preloaded data (e.g., kernel modules) to the
  *      system.  Currently only supported on platforms that create a
  *      vm_phys segment for preloaded data.