MFC r256995:
Remove 128KB bzero() call done for every block I/O data buffer.
MFC r257054:
Some microoptimizations for da and ada drivers:
 - Replace ordered_tag_count counter with single flag;
 - From da remove outstanding_cmds counter, duplicating pending_ccbs list;
 - From da_softc remove unused links field.
MFC r260407:
Allow delete_method sysctl to be set to "DISABLE".