gfs2: use check_submounts_and_drop()

Do have_submounts(), shrink_dcache_parent() and d_drop() atomically.

check_submounts_and_drop() can deal with negative dentries and
non-directories as well.

Non-directories can also be mounted on.  And just like directories we don't
want these to disappear with invalidation.
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
CC: Steven Whitehouse <swhiteho@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/gfs2/dentry.c

@@ -93,12 +93,9 @@ static int gfs2_drevalidate(struct dentry *dentry, unsigned int flags)
 	if (!had_lock)
 		gfs2_glock_dq_uninit(&d_gh);
 invalid:
-	if (inode && S_ISDIR(inode->i_mode)) {
-		if (have_submounts(dentry))
-			goto valid;
-		shrink_dcache_parent(dentry);
-	}
-	d_drop(dentry);
+	if (check_submounts_and_drop(dentry) != 0)
+		goto valid;
+
 	dput(parent);
 	return 0;