target/iscsi: Fix double free in lio_target_tiqn_addtpg()

This iscsit_tpg_add_portal_group() function is only called from
lio_target_tiqn_addtpg().  Both functions free the "tpg" pointer on
error so it's a double free bug.  The memory is allocated in the caller
so it should be freed in the caller and not here.

Fixes: e48354ce ("iscsi-target: Add iSCSI fabric support for target v4.1")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: David Disseldorp <ddiss@suse.de>
[ bvanassche: Added "Fix" at start of patch title ]
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>