Here is a diff to pass around the size to free(9) in amd64's _bus_dmamap allocations.
i386 is symmetric but I don't have the hardware currently to test it. Index: bus_dma.c =================================================================== RCS file: /cvs/src/sys/arch/amd64/amd64/bus_dma.c,v retrieving revision 1.48 diff -u -p -r1.48 bus_dma.c --- bus_dma.c 27 Jan 2015 05:10:30 -0000 1.48 +++ bus_dma.c 17 Dec 2015 11:16:34 -0000 @@ -149,8 +149,11 @@ _bus_dmamap_create(bus_dma_tag_t t, bus_ void _bus_dmamap_destroy(bus_dma_tag_t t, bus_dmamap_t map) { - - free(map, M_DEVBUF, 0); + size_t mapsize; + + mapsize = sizeof(struct bus_dmamap) + + (sizeof(bus_dma_segment_t) * (map->_dm_segcnt - 1)); + free(map, M_DEVBUF, mapsize); } /*