bcachefs: optimize __bch2_trans_get(), kill DEBUG_TRANSACTIONS

 - Some tweaks to greatly reduce locking overhead for the list of btree
   transactions, so that it can always be enabled: leave btree_trans
   objects on the list when they're on the percpu single item freelist,
   and only check for duplicates in the same process when
   CONFIG_BCACHEFS_DEBUG is enabled

 - don't zero out the full btree_trans() unless we allocated it from
   the mempool

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>