switch dentry_open() to struct path, make it grab references itself

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 372ec65..e423f5f 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2157,8 +2157,7 @@
 						get_file(devnull);
 					} else {
 						devnull = dentry_open(
-							dget(selinux_null),
-							mntget(selinuxfs_mount),
+							&selinux_null,
 							O_RDWR, cred);
 						if (IS_ERR(devnull)) {
 							devnull = NULL;