scsi-disk: Move active request asserts

SCSI read/write requests should not be re-issued before the current
fragment of I/O completes.  There are asserts in scsi-disk.c that guard
this constraint but they trigger on SPARC Linux 2.4.  It turns out that
the asserts are too early in the code path and don't allow for read
requests to terminate.

Only the read assert needs to be moved but move the write assert too for
consistency.

Reported-by: Nigel Horne <njh@bandsman.co.uk>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
