scsi: target: cxgbit: Unmap DMA buffer before calling target_execute_cmd()

Instead of calling dma_unmap_sg() after completing WRITE I/O, call
dma_unmap_sg() before calling target_execute_cmd() to sync the DMA buffer.

Link: https://lore.kernel.org/r/1618403949-3443-1-git-send-email-varun@chelsio.com
Cc: <stable@vger.kernel.org> # 5.4+
Signed-off-by: Varun Prakash <varun@chelsio.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>