Currently pointer tdata is being dereferenced on the initialization of
pointer skb before tdata is null checked. This could lead to a potential
null pointer dereference.  Fix this by dereferencing tdata after tdata has
been null pointer sanity checked.

Link: https://lore.kernel.org/r/20200709135217.1408105-1-colin.king@canonical.com
Fixes: e33c2482 ("scsi: cxgb4i: Add support for iSCSI segmentation offload")
Addresses-Coverity: ("Dereference before null check")
Acked-by: Varun Prakash <varun@chelsio.com>
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>