osd/PG.cc: silence "-Wsign-compare" warnings

osd/PG.cc: silence "-Wsign-compare" warnings

/ceph/src/osd/PG.cc: In member function 'void
PG::choose_async_recovery_ec(const std::map<pg_shard_t, pg_info_t>&,
const pg_info_t&, std::vector<int>*, std::set<pg_shard_t>*) const':
/ceph/src/osd/PG.cc:1572:32: warning: comparison of integer expressions
of different signedness: 'long int' and 'long unsigned int'
[-Wsign-compare]
     if (approx_missing_objects > cct->_conf.get_val<uint64_t>(
         ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
         "osd_async_recovery_min_cost")) {
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/ceph/src/osd/PG.cc: In member function 'void
PG::choose_async_recovery_replicated(const std::map<pg_shard_t,
pg_info_t>&, const pg_info_t&, std::vector<int>*, std::set<pg_shard_t>*)
const':
/ceph/src/osd/PG.cc:1625:33: warning: comparison of integer expressions
of different signedness: 'long int' and 'long unsigned int'
[-Wsign-compare]
     if (approx_missing_objects  > cct->_conf.get_val<uint64_t>(
         ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
         "osd_async_recovery_min_cost")) {
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Kefu Chai <kchai@redhat.com>