Don't use cmd->request->nr_hw_segments as it may not be initialized
  (SG_IO in particular bypasses anything that initializes this and just
  uses scsi_do_req to insert a scsi_request directly on the head of the
  queue) and a bogus value here can trip up the checks to make sure that
  the number of segments will fit in the queue ring buffer, resulting in
  commands that are never completed.

  Fix up several issues with PCI DMA mapping and failure to check return
  values on the mappings.

  Make the check for space in the ring buffer happen after the DMA mapping
  is done since any checks done before the mapping has taken place are
  bogus.

  Doug Ledford <dledford@redhat.com>.
Signed-off-by: Andrew Vasquez <andrew.vasquez@qlogic.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>