target_setup_cmd_from_cdb() is called after a successful call to
transport_lookup_cmd_lun(). The new helper factors out the code that can be
called before the call to transport_lookup_cmd_lun(). This helper will be
used in an upcoming commit to address NULL pointer dereference.

Link: https://lore.kernel.org/r/1591559913-8388-2-git-send-email-sudhakar.panneerselvam@oracle.comReviewed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Sudhakar Panneerselvam <sudhakar.panneerselvam@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>