jffs2: Add missing capability check for listing trusted xattrs

The vfs checks if a task has the appropriate access for get and set
operations, but it cannot do that for the list operation; the file system
must check for that itself.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: linux-mtd@lists.infradead.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>