libata: only ports >= 0 need to synchronize

In a discussio with Jeff Garzik, he mentioned that the serialization
for the libata port probes only needs to be within the domain of a host.
This means that for the first port of each host (with ID 0), we don't
need to wait, so we can relax our serialization a little.
Signed-off-by: NArjan van de Ven <arjan@linux.intel.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/ata/libata-core.c

@@ -5925,8 +5925,10 @@ static void async_port_probe(void *data, async_cookie_t cookie)
 	 * If we're not allowed to scan this host in parallel,
 	 * we need to wait until all previous scans have completed
 	 * before going further.
+	 * Jeff Garzik says this is only within a controller, so we
+	 * don't need to wait for port 0, only for later ports.
 	 */
-	if (!(ap->host->flags & ATA_HOST_PARALLEL_SCAN))
+	if (!(ap->host->flags & ATA_HOST_PARALLEL_SCAN) && ap->port_no != 0)
 		async_synchronize_cookie(cookie);
 
 	/* probe */