ceph: don't list vxattrs in listxattr()

Most filesystems that provide virtual xattrs (e.g. CIFS) don't display
them via listxattr(). Ceph does, and that causes some of the tests in
xfstests to fail.

Have cephfs stop listing vxattrs in listxattr. Userspace can always
query them directly when the name is known.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Acked-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>