mon/OSDMonitor: refix warning

mon/OSDMonitor.cc: In member function 'int OSDMonitor::_prepare_rename_pool(int64_t, std::string)':
mon/OSDMonitor.cc:4370:55: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

Two fixes for this crossed paths in
0c186096bb9534165394046fd8dd914e1be44094 and
2181b4c94696432814fea828a7efa54586b324a4

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