libata: prevent devices with blank model names from being DMA blacklisted

The strn_pattern_cmp routine does not handle a blank name parameter
properly. The only patterns which should match a blank name are "*"
and an explicit "". If the function is passed a blank name in current
code, it will always match against the patt parameter. The bug manifests
itself as the device with the empty model name always matching the first
device in the DMA blacklist, forcing it to revert to PIO mode.
Signed-off-by: NAndrew Paprocki <andrew@ishiboo.com>
Signed-off-by: NJeff Garzik <jgarzik@redhat.com>

drivers/ata/libata-core.c

@@ -4014,8 +4014,14 @@ int strn_pattern_cmp(const char *patt, const char *name, int wildchar)
 	p = strchr(patt, wildchar);
 	if (p && ((*(p + 1)) == 0))
 		len = p - patt;
-	else
+	else {
 		len = strlen(name);
+		if (!len) {
+			if (!*patt)
+				return 0;
+			return -1;
+		}
+	}
 
 	return strncmp(patt, name, len);
 }