Linux 2.6.x compat, blkdev_compat.h

For legacy reasons the zvol.c and vdev_disk.c Linux compatibility
code ended up in sys/blkdev.h and sys/vdev_disk.h headers.  While
there are worse places for this code to live it should be in a
linux/blkdev_compat.h header.  This change moves this block device
Linux compatibility code in to the linux/blkdev_compat.h header
and updates all the correct #include locations.  This is not a
functional change or bug fix, it is just code cleanup.