scsi: resolve some missing-field-initializers warnings

Resolve some missing-field-initializers warnings by using
designated initialization.

[hch: W=2 with modern gcc warns about this.  Pretty pointless to me, but
 I'd prefer to keep us warning free]
Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/scsi/scsi_lib.c

@@ -47,7 +47,7 @@ struct scsi_host_sg_pool {
 	mempool_t	*pool;
 };
 
-#define SP(x) { x, "sgpool-" __stringify(x) }
+#define SP(x) { .size = x, "sgpool-" __stringify(x) }
 #if (SCSI_MAX_SG_SEGMENTS < 32)
 #error SCSI_MAX_SG_SEGMENTS is too small (must be 32 or greater)
 #endif