scsi: qla4xxx: Fix missing DMA mapping error in qla4xxx_alloc_pdu()

dma_map_XXX() can fail and should be tested for errors with
dma_mapping_error().

Fixes: b3a271a9 ("[SCSI] qla4xxx: support iscsiadm session mgmt")
Signed-off-by: Thomas Fourier <fourier.thomas@gmail.com>
Link: https://lore.kernel.org/r/20250618071742.21822-2-fourier.thomas@gmail.com


Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>