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>

tests/generic/020

@@ -154,6 +154,12 @@ _attr_get_maxval_size()
 			awk -F "=0x" '/^user/ {len += length($1) + length($2) / 2} END {print len}')
 		local selinux_size=$(getfattr -n 'security.selinux' --dump -e hex $filename 2>/dev/null | \
 			awk -F "=0x" '/^security/ {len += length($1) + length($2) / 2} END {print len}')
+		if [ -z $size ]; then
+			size=0
+		fi
+		if [ -z $selinux_size ]; then
+			selinux_size=0
+		fi
 		max_attrval_size=$((65536 - $size - $selinux_size - $max_attrval_namelen))
 		;;
 	*)