DA_CCB_TRIM completion decrements outstanding_cmds_rd that the TRIM start path never increments: permanent queue-accounting underflow disables read throttling and tps balancing

Summary

dadone handles BUFFER_IO and TRIM in one shared block whose epilogue decrements the rd/wr counters, but the TRIM issuance path breaks out BEFORE the increment - every completed TRIM permanently decrements outstanding_cmds_rd: read throttle permanently satisfied (queued-read depth unbounded), tps_rd normalization can never fire again, write starvation under mixed load after the FIRST trim. Reachable unpriv via filesystem trims (ffs_blkfree_trim/spec_freeblks) on trim-enabled da disks. Skip-decrement-for-TRIM fix.