ubifs: Use NULL instead of using plain integer as pointer

This fixes the following sparse warnings:
fs/ubifs/xattr.c:680:58: warning: Using plain integer as NULL pointer
Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

fs/ubifs/xattr.c

@@ -677,7 +677,7 @@ int ubifs_init_security(struct inode *dentry, struct inode *inode,
 	int err;
 
 	err = security_inode_init_security(inode, dentry, qstr,
-					   &init_xattrs, 0);
+					   &init_xattrs, NULL);
 	if (err) {
 		struct ubifs_info *c = dentry->i_sb->s_fs_info;
 		ubifs_err(c, "cannot initialize security for inode %lu, error %d",