sysfs: Do not return POSIX ACL xattrs via listxattr

Commit 786534b92f3c introduced a regression that caused listxattr to
return the POSIX ACL attribute names even though sysfs doesn't support
POSIX ACLs. This happens because simple_xattr_list checks for NULL
i_acl / i_default_acl, but inode_init_always initializes those fields
to ACL_NOT_CACHED ((void *)-1). For example:
$ getfattr -m- -d /sys
/sys: system.posix_acl_access: Operation not supported
/sys: system.posix_acl_default: Operation not supported
Fix this in simple_xattr_list by checking if the filesystem supports POSIX ACLs.

Fixes: 786534b92f3c ("tmpfs: listxattr should include POSIX ACL xattrs")
Reported-by: Marc Aurèle La France <[e-mail]>
Tested-by: Marc Aurèle La France <[e-mail]>
Signed-off-by: Andreas Gruenbacher <[e-mail]>
Cc: [e-mail] # v4.5+
Signed-off-by: Al Viro <[e-mail]>

fs/xattr.c

···
 	int err = 0;
 
 #ifdef CONFIG_FS_POSIX_ACL
-	if (inode->i_acl) {
-		err = xattr_list_one(&buffer, &remaining_size,
-				     XATTR_NAME_POSIX_ACL_ACCESS);
-		if (err)
-			return err;
-	}
-	if (inode->i_default_acl) {
-		err = xattr_list_one(&buffer, &remaining_size,
-				     XATTR_NAME_POSIX_ACL_DEFAULT);
-		if (err)
-			return err;
+	if (IS_POSIXACL(inode)) {
+		if (inode->i_acl) {
+			err = xattr_list_one(&buffer, &remaining_size,
+					     XATTR_NAME_POSIX_ACL_ACCESS);
+			if (err)
+				return err;
+		}
+		if (inode->i_default_acl) {
+			err = xattr_list_one(&buffer, &remaining_size,
+					     XATTR_NAME_POSIX_ACL_DEFAULT);
+			if (err)
+				return err;
+		}
 	}
 #endif