test/bufferlist: fix warning

In file included from test/bufferlist.cc:31:0:
../src/gtest/include/gtest/gtest.h: In function ‘testing::AssertionResult testing::internal::CmpHelperEQ(const char*, const char*, const T1&, const T2&) [with T1 = unsigned int, T2 = int]’:
../src/gtest/include/gtest/gtest.h:1300:30: instantiated from ‘static testing::AssertionResult testing::internal::EqHelper::Compare(const char*, const char*, const T1&, const T2&) [with T1 = unsigned int, T2 = int, bool lhs_is_null_literal = false]’
test/bufferlist.cc:1604:227: instantiated from here
warning: ../src/gtest/include/gtest/gtest.h:1263:3: comparison between signed and unsigned integer expressions [-Wsign-compare]

Signed-off-by: Sage Weil <sage@inktank.com>