osd: correct unsigned/signed compiler wrn

osd: correct unsigned/signed compiler wrn

    /home/pdonnell/ceph/src/osd/OSD.cc: In member function ‘void OSD::ShardedOpWQ::stop_for_fast_shutdown()’:
    /home/pdonnell/ceph/src/osd/OSD.cc:11143:41: warning: comparison of integer expressions of different signedness: ‘int’ and ‘uint32_t’ {aka ‘unsigned int’} [-Wsign-compare]
    11143 |   for (int shard_index = 0; shard_index < osd->num_shards; shard_index++) {

Fixes: https://tracker.ceph.com/issues/62851
Fixes: 210dbd4ff19ea66fd2f0109cc15aad53349be52f
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>