target/tcm_qla2xxx: Add/use target_reverse_dma_direction() in target_core_fabric.h

Reversing the dma_data_direction for pci_map_sg() friends is useful
for other drivers, so move it from tcm_qla2xxx into inline code
within target_core_fabric.h.

Also drop internal usage of equivlient in tcm_qla2xxx fabric code.

Reported-by: Christoph Hellwig <hch@lst.de>
Cc: Roland Dreier <roland@purestorage.com>
Cc: Giridhar Malavali <giridhar.malavali@qlogic.com>
Cc: Chad Dupuis <chad.dupuis@qlogic.com>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Nicholas Bellinger <nab@daterainc.com>