crimson/os: use reference for loop variable

crimson/os: use reference for loop variable

for better performance, also silences the warning like:

../src/crimson/os/seastore/random_block_manager/nvme_manager.cc:444:23: warning: loop variable ‘b’ creates a copy from type ‘const crimson::os::seastore::rbm_alloc_delta_t’ [-Wrange-loop-construct]
  444 |       for (const auto b : alloc_blocks) {
      |                       ^

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