libata-scsi: improve rbuf handling for simulated commands

Buffer length handling in simulated commands is error-prone and full
of bugs.  There are a number of places where necessary length checks
are missing and if the output buffer is passed in as sglist, nothing
works.

This patch adds a static buffer ata_scsi_rbuf which is sufficiently
large to handle the larges output from simulated commands (4k
currently), let all simulte functions write to the buffer and removes
all length checks as we know that there always is enough buffer space.
Copying in (for ATAPI inquiry fix up) and out are handled by
sg_copy_to/from_buffer() behind ata_scsi_rbuf_get/put() interface
which handles sglist properly.

This patch is inspired from buffer length check fix patch from Petr
Vandrovec.

Updated to use sg_copy_to/from_buffer() as suggested by FUJITA
Tomonori.
Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Petr Vandrovec <petr@vmware.com>
Cc: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>