[arbel] Allocate sufficient space for firmware buffer

We were accidentally allocating only half the required amount of
memory (given the alignment method) for the firmware buffer, leading
to conflicts between the firmware buffer and gPXE code/data segments.

src/drivers/infiniband/arbel.c

@@ -1714,7 +1714,7 @@ static int arbel_start_firmware ( struct arbel *arbel ) {
 
 	/* Allocate firmware pages and map firmware area */
 	fw_size = ( fw_pages * 4096 );
-	arbel->firmware_area = umalloc ( fw_size );
+	arbel->firmware_area = umalloc ( fw_size * 2 );
 	if ( ! arbel->firmware_area ) {
 		rc = -ENOMEM;
 		goto err_alloc_fa;