MFC: r286785, r291088, r291120

• Reformat x86 bounce buffer synchronization code to reduce indentation. No functional change.
• Avoid a NULL pointer dereference in bounce_bus_dmamap_sync() when the map has been created via bounce_bus_dmamem_alloc(). Even for coherent DMA - which bus_dmamem_alloc(9) typically is used for -, calling of bus_dmamap_sync(9) isn't optional. [1]
• Avoid a NULL pointer dereference in bounce_bus_dmamap_unload() when the map has been created via bounce_bus_dmamem_alloc(). In that case bus_dmamap_unload(9) typically isn't called during normal operation but still should be during detach, cleanup from failed attach etc. [2]

PR: 188899 (non-original problem) [1]
Submitted by: yongari [2]