Fix sign-compare warnings

system/core/fs_mgr/fs_mgr_fstab.cpp:161:18: error: comparison of integers of different signs: 'int' and 'const uint64_t' (aka 'const unsigned long') [-Werror,-Wsign-compare]
        if (flag == j->flag) {
            ~~~~ ^  ~~~~~~~

system/core/fs_mgr/fs_mgr.cpp:674:11: error: comparison of integers of different signs: 'int' and 'std::__1::vector<FstabEntry, std::__1::allocator<FstabEntry> >::size_type' (aka 'unsigned long') [-Werror,-Wsign-compare]
    if (i < fstab.size()) --i;
        ~ ^ ~~~~~~~~~~~~

Test: m checkbuild
Change-Id: I056add222a92f99134a49ac141bd143ddd54179b