scsi: tcm_qla2xxx: Make qlt_alloc_qfull_cmd() set cmd->se_cmd.map_tag

Reduce code duplication by introducing the tcm_qla2xxx_{get,rel}_cmd()
functions. Introduce these functions in the tcm_qla2xxx.c source files such
that the qla_target.c source file becomes independent of the SCSI target
core header files. This patch fixes a bug, namely by ensuring that
qlt_alloc_qfull_cmd() sets cmd->se_cmd.map_tag.

Cc: Himanshu Madhani <hmadhani@marvell.com>
Link: https://lore.kernel.org/r/20200103192719.205158-1-bvanassche@acm.org
Acked-by: Himanshu Madhani <hmadhani@marvell.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>