target: Fix a memory leak in target_dev_lba_map_store()

strsep() modifies its first argument. Make the pointer passed to kfree()
match the return value of kmalloc().

Fixes: 229d4f11 (commit "target_core_alua: Referrals configfs integration")
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Andy Grover <agrover@redhat.com>
Cc: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>