With some recent busdma refactoring, sometimes it happens that a sync
op gets called when bus_dmamap_load() never got called, which results
in a spurious warning about a map mismatch when no sync operations will
actually happen anyway. Now the check is done only if a sync operation
is actually performed, and the result of the check is a panic, not just
a printf.
Reviewed by: cognet (who prevented me from donning a point hat)