scsi: libcxgbi: use GFP_ATOMIC in cxgbi_conn_alloc_pdu()

For mgmt cmds ->alloc_pdu() can be called from atomic context so use
GFP_ATOMIC instead of GFP_KERNEL.

Signed-off-by: Varun Prakash <varun@chelsio.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>