[SCSI] st: make all the fragment buffers the same size

This patch simiplifies the fragment buffer management a bit, all the
buffers in the fragment list become the same size. This is necessary
to use the block layer API (sg driver was modified in the same way)
since the block layer API takes the same size page frames instead of
scatter gatter.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Kai Makisara <Kai.Makisara@kolumbus.fi>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>