osd/scrubber: fix signed comparison warning

osd/scrubber: fix signed comparison warning

[681/1140] Building CXX object src/osd/CMakeFiles/osd.dir/scrubber/scrub_resources.cc.o
src/osd/scrubber/scrub_resources.cc: In member function ‘bool Scrub::ScrubResources::inc_scrubs_remote(pg_t)’:
src/osd/scrubber/scrub_resources.cc:84:18: warning: comparison of integer expressions of different signedness: ‘long unsigned int’ and ‘const int64_t’ {aka ‘const long int’} [-Wsign-compare]
   84 |   if (pre_op_cnt < conf->osd_max_scrubs) {
      |       ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Casey Bodley <cbodley@redhat.com>