MFC: r286785, r291088, r291120

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]

git-svn-id: svn://svn.freebsd.org/base/stable/10@292775 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f