osdc: Fix compiler warning

Was getting:

./osdc/ObjectCacher.h:169:51: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
       uint64_t overlap_start = MAX(start, ex.start);
                                                   ^
./osdc/ObjectCacher.h:170:59: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
       uint64_t overlap_end = MIN(end, ex.start + ex.length);
                                                           ^

Signed-off-by: Adam Crume <adamcrume@gmail.com>
Merge pull request #2746 from ceph/wip-adamcrume

osdc: Fix compiler warning

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Loic Dachary <loic-201408@dachary.org>