[SCSI] Enable clustering and large transfers

This patch enables clustering and sets max_sectors to 0xffff to enable
reading and writing of large blocks with tapes (and large transfers with
sg). This change is needed after the sg and st drivers started using
chained bios through scsi_request_async() in 2.6.16.

Signed-off-by: Kai Makisara <kai.makisara@kolumbus.fi>
Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>