sysfs: Do not return POSIX ACL xattrs via listxattr

Commit 786534b9 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: 786534b9 ("tmpfs: listxattr should include POSIX ACL xattrs")
Reported-by: Marc Aurèle La France <tsi@tuyoix.net>
Tested-by: Marc Aurèle La France <tsi@tuyoix.net>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Cc: stable@vger.kernel.org # v4.5+
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/xattr.c

@@ -948,6 +948,7 @@ ssize_t simple_xattr_list(struct inode *inode, struct simple_xattrs *xattrs,
 	int err = 0;
 
 #ifdef CONFIG_FS_POSIX_ACL
+	if (IS_POSIXACL(inode)) {
 		if (inode->i_acl) {
 			err = xattr_list_one(&buffer, &remaining_size,
 					     XATTR_NAME_POSIX_ACL_ACCESS);
@@ -960,6 +961,7 @@ ssize_t simple_xattr_list(struct inode *inode, struct simple_xattrs *xattrs,
 			if (err)
 				return err;
 		}
+	}
 #endif
 
 	spin_lock(&xattrs->lock);