xfrm: make aead_len() return unsigned int

Key lengths can't be negative.

Comparison with nla_len() is left signed just in case negative value
can sneak in there.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
diff --git a/include/net/xfrm.h b/include/net/xfrm.h
index f002a2c..0be4c54 100644
--- a/include/net/xfrm.h
+++ b/include/net/xfrm.h
@@ -1764,7 +1764,7 @@
 }
 #endif
 
-static inline int aead_len(struct xfrm_algo_aead *alg)
+static inline unsigned int aead_len(struct xfrm_algo_aead *alg)
 {
 	return sizeof(*alg) + ((alg->alg_key_len + 7) / 8);
 }