[SCSI] sg: fix a bug in st_map_user_pages failure path

sg's st_map_user_pages is modelled on an earlier version of st's
sgl_map_user_pages, and has the same bug: if get_user_pages got some but
not all of the pages, then those got were released, but the positive res
code returned implied that they were still to be freed.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Douglas Gilbert <dougg@torque.net>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>