scsi: explicitly release bidi buffers

Instead of trying to guess when we have a BIDI buffer in scsi_release_buffers
add a function to explicitly free the BIDI ressoures in the one place that
handles them.  This avoids needing a special __scsi_release_buffers for the
case where we already have freed the request as well.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mike Christie <michaelc@cs.wisc.edu>