crimson/osd: use reference when structured binding

crimson/osd: use reference when structured binding

to silence warning from clang13 with -std=c++20, like

ceph/src/crimson/osd/replicated_recovery_backend.cc:1098:21: warning: loop variable '[off, len]' creates a copy from type 'const std::pair<const unsigned long, unsigned long>' [-Wrange-loop-construct]
    for (const auto [off, len] : extents) {
                    ^
ceph/src/crimson/osd/replicated_recovery_backend.cc:1098:10: note: use reference type 'const std::pair<const unsigned long, unsigned long> &' to prevent copying
    for (const auto [off, len] : extents) {
         ^~~~~~~~~~~~~~~~~~~~~~~
                    &

Signed-off-by: Kefu Chai <tchaikov@gmail.com>