[SCSI] add shared tag map helpers

This patch adds support for sharing tag maps at the host level
(i.e. either every queue [LUN] has its own tag map or there's a single
one for the entire host).  This formulation is primarily intended to
help single issue queue hardware, like the aic7xxx

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>