mmu: Fix signed and unsigned integer comparison error

  CC bsd/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zvol.o
  CXX core/mmu.o
/home/zwu/osv/core/mmu.cc: In instantiation of ‘void mmu::map_level<PageOp, ParentLevel>::operator()(mmu::hw_ptep, uintptr_t, uintptr_t) [with PageOp = mmu::virt_to_phys_map; int ParentLevel = 4; uintptr_t = long unsigned int]’:
/home/zwu/osv/core/mmu.cc:323:5:   required from ‘void mmu::map_range(uintptr_t, size_t, PageOp&, size_t) [with PageOp = mmu::virt_to_phys_map; uintptr_t = long unsigned int; size_t = long unsigned int]’
/home/zwu/osv/core/mmu.cc:623:43:   required from here
/home/zwu/osv/core/mmu.cc:383:13: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
...

Signed-off-by: Zhi Yong Wu <zwu.kernel@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

core/mmu.cc

@@ -380,7 +380,7 @@ private:
             hw_ptep ptep = pt.at(idx);
             uintptr_t vstart1 = vcur, vend1 = vend;
             clamp(vstart1, vend1, base_virt, base_virt + step - 1, slop);
-            if (level < nr_page_sizes && vstart1 == base_virt && vend1 == base_virt + step - 1) {
+            if (unsigned(level) < nr_page_sizes && vstart1 == base_virt && vend1 == base_virt + step - 1) {
                 uintptr_t offset = base_virt - vstart;
                 if (level) {
                     if (!skip_pte(ptep)) {