target/pscsi: call spc_emulate_report_luns directly

No need to indirect through spc_parse_cdb if we only ever call it for
REPORT LUNS emulation.

(nab: Add missing EXPORT_SYMBOL for spc_emulate_report_luns)

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>