reiserfs: minimal fix for ACL caching

reiserfs uses NULL as "unknown" and ERR_PTR(-ENODATA) as "no ACL";
several codepaths store the former instead of the latter.

All those codepaths go through iset_acl() and all cases when it's
called with NULL acl are for the second variety, so the minimal
fix is to teach iset_acl() to deal with that.

Proper fix is to switch to more usual conventions and avoid back
and forth between internally used ERR_PTR(-ENODATA) and NULL
expected by the rest of the kernel.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/reiserfs/xattr_acl.c

@@ -194,7 +194,7 @@ static inline void iset_acl(struct inode *inode, struct posix_acl **i_acl,
 	spin_lock(&inode->i_lock);
 	if (*i_acl != ERR_PTR(-ENODATA))
 		posix_acl_release(*i_acl);
-	*i_acl = posix_acl_dup(acl);
+	*i_acl = acl ? posix_acl_dup(acl) : ERR_PTR(-ENODATA);
 	spin_unlock(&inode->i_lock);
 }