MFC r269123:
Implement separate I/O dispatch method for ZVOLs in "dev" mode.

Unlike disk devices ZVOLs process all requests synchronously.  That makes
impossible sending multiple requests to them from single thread.  From the
other side ZVOLs have real d_read/d_write methods, which unlike d_strategy
can handle uio scatter/gather and have no strict I/O size limitations.

So, if ZVOL in "dev" mode is detected, use of d_read/d_write methods instead
of d_strategy allows to avoid pointless splitting of large requests into
MAXPHYS (128K) sized chunks.