target: Fix user data segment multiplier in spc_emulate_evpd_b3()

This patch fixes an apparent cut and paste error in spc_emulate_evpd_b3(),
where lba_map_segment_size was being used twice for the Referrals VPD.

Go ahead and set the correct user data segment multiplier instead of
user data segment size.

Signed-off-by: Sebastian Herbszt <herbszt@gmx.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>