abis_nm: fix -Wunused-but-set-variable (bug)

clang warns us about 'len' being set, but not used:  And this is

  abis_nm.c:2172:10: warning: variable 'len' set but not used [-Wunused-but-set-variable]
        uint8_t len = attr_len;
                ^

This is actually a bug, because in the case of NACK we append 2 more
bytes {NM_ATT_NACK_CAUSES, NM_NACK_OBJCLASS_NOTSUPP}, and we need to
pass the final length to fill_om_fom_hdr(), including those optional
two bytes.  Passing 'attr_len' (length of 'attr') is wrong.

Change-Id: I3ca8e761fdf99dd498a979ccc9d53c6c3e03e2cc
---
M src/osmo-bsc/abis_nm.c
1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/src/osmo-bsc/abis_nm.c b/src/osmo-bsc/abis_nm.c
index 7a9789b..afb7abc 100644
--- a/src/osmo-bsc/abis_nm.c
+++ b/src/osmo-bsc/abis_nm.c
@@ -2183,7 +2183,7 @@
 	}
 
 	oh = (struct abis_om_hdr *) msgb_put(msg, ABIS_OM_FOM_HDR_SIZE);
-	fill_om_fom_hdr(oh, attr_len, msgtype, obj_class, i1, i2, i3);
+	fill_om_fom_hdr(oh, len, msgtype, obj_class, i1, i2, i3);
 
 	if (attr != NULL && attr_len > 0)
 		memcpy(msgb_put(msg, attr_len), attr, attr_len);

