target_core_alua: Allocate ALUA metadata on demand

We should only allocate ALUA metadata if we're actually going
to write them.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

diff --cc drivers/target/target_core_alua.c
Simple merge

diff --cc drivers/target/target_core_alua.h
Simple merge

diff --cc include/target/target_core_base.h
Simple merge