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

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>