scsi: libiscsi: Initialize iscsi_conn->dd_data only if memory is allocated

[ Upstream commit 3ea3a256ed81f95ab0f3281a0e234b01a9cae605 ]

In case of an ib_fast_reg_mr allocation failure during iSER setup, the
machine hits a panic because iscsi_conn->dd_data is initialized
unconditionally, even when no memory is allocated (dd_size == 0).  This
leads invalid pointer dereference during connection teardown.

Fix by setting iscsi_conn->dd_data only if memory is actually allocated.

Panic trace:
------------
 iser: iser_create_fastreg_desc: Failed to allocate ib_fast_reg_mr err=-12
 iser: iser_alloc_rx_descriptors: failed allocating rx descriptors / data buffers
 BUG: unable to handle page fault for address: fffffffffffffff8
 RIP: 0010:swake_up_locked.part.5+0xa/0x40
 Call Trace:
  complete+0x31/0x40
  iscsi_iser_conn_stop+0x88/0xb0 [ib_iser]
  iscsi_stop_conn+0x66/0xc0 [scsi_transport_iscsi]
  iscsi_if_stop_conn+0x14a/0x150 [scsi_transport_iscsi]
  iscsi_if_rx+0x1135/0x1834 [scsi_transport_iscsi]
  ? netlink_lookup+0x12f/0x1b0
  ? netlink_deliver_tap+0x2c/0x200
  netlink_unicast+0x1ab/0x280
  netlink_sendmsg+0x257/0x4f0
  ? _copy_from_user+0x29/0x60
  sock_sendmsg+0x5f/0x70

Signed-off-by: Showrya M N <showrya@chelsio.com>
Signed-off-by: Potnuri Bharat Teja <bharat@chelsio.com>
Link: https://lore.kernel.org/r/20250627112329.19763-1-showrya@chelsio.com


Reviewed-by: Chris Leech <cleech@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>