[SCSI] osst: make all the buffer the same size

This simiplifies the buffer management; all the buffers in osst_buffer
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>
Cc: Willem Riede <osst@riede.org>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>