fix a leak in replace_fd() users

replace_fd() began with "eats a reference, tries to insert into
descriptor table" semantics; at some point I'd switched it to
much saner current behaviour ("try to insert into descriptor
table, grabbing a new reference if inserted; caller should do
fput() in any case"), but forgot to update the callers.
Mea culpa...

[Spotted by Pavel Roskin, who has really weird system with pipe-fed
coredumps as part of what he considers a normal boot ;-)]

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 24ab414..61a5336 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2132,18 +2132,14 @@
 		return;
 
 	devnull = dentry_open(&selinux_null, O_RDWR, cred);
-	if (!IS_ERR(devnull)) {
-		/* replace all the matching ones with this */
-		do {
-			replace_fd(n - 1, get_file(devnull), 0);
-		} while ((n = iterate_fd(files, n, match_file, cred)) != 0);
+	if (IS_ERR(devnull))
+		devnull = NULL;
+	/* replace all the matching ones with this */
+	do {
+		replace_fd(n - 1, devnull, 0);
+	} while ((n = iterate_fd(files, n, match_file, cred)) != 0);
+	if (devnull)
 		fput(devnull);
-	} else {
-		/* just close all the matching ones */
-		do {
-			replace_fd(n - 1, NULL, 0);
-		} while ((n = iterate_fd(files, n, match_file, cred)) != 0);
-	}
 }
 
 /*