[SCSI] introduce shost_priv helper

Currently accessing the scsi host private data is rather messy because
it comes as an unsigned long that always needs a cast first.  This patch
introduces a helper that does the cast called shost_priv.  It's similar
in spirit to netdev_priv for network drivers.

This is the first patch introducing the macro, and the second patch
in the series will convert esp and it's subdrivers as an example.
Further conversion will wait until the helper is in the tree to make
patch juggling easier.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>