crimson,mgr: mark assert-only variables [[maybe_unused]]

These variables are only read inside assert(), which is compiled out
under NDEBUG. Mark them [[maybe_unused]] to silence the warnings while
keeping the debug-only assert() style used by the surrounding code:

  src/crimson/os/seastore/lba/btree_lba_manager.cc:1078: unused variable 'orig_len' [-Wunused-variable]
  src/crimson/os/seastore/omap_manager/log/log_manager.cc:73: variable 'ret' set but not used [-Wunused-but-set-variable]
  src/crimson/os/seastore/transaction_manager.cc:382: variable 'intermediate_key' set but not used [-Wunused-but-set-variable]
  src/mgr/PyModule.cc:166,186: unused variable 'r' [-Wunused-variable]

Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>