mon&test: resolve warning about inconsistent variable types

mon&test: resolve warning about inconsistent variable types

resolve a warning issue about inconsistent variable types with
Building CXX object src/mon/CMakeFiles/mon.dir/Elector.cc.o
../src/mon/Elector.cc: In member function 'void Elector::
notify_rank_removed(int)':
../src/mon/Elector.cc:765:15: warning: comparison of integer
expressions of different signedness: 'int' and 'unsigned int'
[-Wsign-compare] if (rank_removed < paxos_size()) {
warning:comparison of integer expressions of different
signedness: 'unsigned int' and 'const int'
[-Wsign-compare] if (total < NR_ROUNDS)

Fixes: https://tracker.ceph.com/issues/63917
Signed-off-by: cuiming <cuiming_yewu@cmss.chinamobile.com>