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>
network: fix compile error

BUILD SUCCESSFUL

Total time: 35.396 secs
make -r -C build/release/ all
make[1]: Entering directory `/home/zwu/osv/build/release'
  CXX loader.o
  CXX runtime.o
  CXX drivers/vga.o
  CXX bsd/net.o
  CXX bsd/porting/networking.o
/home/zwu/osv/bsd/porting/networking.cc: In function ‘int osv::if_set_mtu(std::string, u16)’:
/home/zwu/osv/bsd/porting/networking.cc:43:32: error: missing braces around initializer for ‘char [16]’ [-Werror=missing-braces]
cc1plus: all warnings being treated as errors
make[1]: *** [bsd/porting/networking.o] Error 1
make[1]: Leaving directory `/home/zwu/osv/build/release'
make: *** [all] Error 2

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