Improve iser_rx_desc alignment

Align iscsi_header[] and data[] on an 8-byte boundary instead of on a 4-byte
boundary. With this patch applied pahole produces the following output:

struct iser_rx_desc {
        struct iser_hdr    iser_header;                  /*     0    28 */
        char                       pad1[4];              /*    28     4 */
        char                       iscsi_header[48];     /*    32    48 */
        /* --- cacheline 1 boundary (64 bytes) was 16 bytes ago --- */
        char                       data[128];            /*    80   128 */
        /* --- cacheline 3 boundary (192 bytes) was 16 bytes ago --- */
        struct ibv_sge     rx_sg;                        /*   208    16 */
        struct ibv_mr *            hdr_mr;               /*   224     8 */
        char                       pad2[24];             /*   232    24 */

        /* size: 256, cachelines: 4, members: 7 */
};

Additionally, this patch fixes the following build errors:

iser.c: In function 'iser_alloc_rx_descriptors':
iser.c:916:11: error: taking address of packed member of 'struct iser_rx_desc' may result in an unaligned pointer value [-Werror=address-of-packed-member]
  916 |   rx_sg = &rx_desc->rx_sg;
      |           ^~~~~~~~~~~~~~~
iser.c: In function 'iser_post_recvm':
iser.c:955:20: error: taking address of packed member of 'struct iser_rx_desc' may result in an unaligned pointer value [-Werror=address-of-packed-member]
  955 |   rx_wr->sg_list = &rx_desc->rx_sg;
      |                    ^~~~~~~~~~~~~~~

Signed-off-by: Bart Van Assche <bvanassche@acm.org>

include/scsi-lowlevel.h

@@ -30,6 +30,10 @@
 extern "C" {
 #endif
 
+#ifndef static_assert
+#define static_assert(e, m) extern char unused_array[1 - 2 * !(e)]
+#endif
+
 #define SCSI_CDB_MAX_SIZE			16
 
 enum scsi_opcode {