generic/020: fix another really long attr test failure for ceph

generic/020: fix another really long attr test failure for ceph

If the CONFIG_CEPH_FS_SECURITY_LABEL is disabled the kernel ceph
the 'selinux_size' will be empty and then:
max_attrval_size=$((65536 - $size - $selinux_size - $max_attrval_namelen))
will be:
max_attrval_size=$((65536 - $size - - $max_attrval_namelen))
which equals to:
max_attrval_size=$((65536 - $size + $max_attrval_namelen))

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>