gfs2: Minor gfs2_alloc_inode cleanup

In gfs2_alloc_inode, when kmem_cache_alloc cannot allocate a new object, return
NULL immediately.  The code currently relies on the fact that i_inode is the
first member in struct gfs2_inode and so ip and &ip->i_inode evaluate to the
same address, but that isn't immediately obvious.
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Bob Peterson <rpeterso@redhat.com>

fs/gfs2/super.c

@@ -1722,13 +1722,13 @@ static struct inode *gfs2_alloc_inode(struct super_block *sb)
 	struct gfs2_inode *ip;
 
 	ip = kmem_cache_alloc(gfs2_inode_cachep, GFP_KERNEL);
-	if (ip) {
+	if (!ip)
+		return NULL;
 	ip->i_flags = 0;
 	ip->i_gl = NULL;
 	memset(&ip->i_res, 0, sizeof(ip->i_res));
 	RB_CLEAR_NODE(&ip->i_res.rs_node);
 	ip->i_rahead = 0;
-	}
 	return &ip->i_inode;
 }