target: Don't BUG_ON during NodeACL dynamic -> explicit conversion

After the v4.2+ RCU conversion to se_node_acl->lun_entry_hlist,
a BUG_ON() was added in core_enable_device_list_for_node() to
detect when the located orig->se_lun_acl contains an existing
se_lun_acl pointer reference.

However, this scenario can happen when a dynamically generated
NodeACL is being converted to an explicit NodeACL, when the
explicit NodeACL contains a different LUN mapping than the
default provided by the WWN endpoint.

So instead of triggering BUG_ON(), go ahead and fail instead
following the original pre RCU conversion logic.

Reported-by: Benjamin ESTRABAUD <ben.estrabaud@mpstor.com>
Cc: Benjamin ESTRABAUD <ben.estrabaud@mpstor.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: stable@vger.kernel.org # 4.2+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>