net: fix nla_strcmp to handle more then one trailing null character

Android userspace has been using TCA_KIND with a char[IFNAMESIZ]
many-null-terminated buffer containing the string 'bpf'.

This works on 4.19 and ceases to work on 5.10.

I'm not entirely sure what fixes tag to use, but I think the issue
was likely introduced in the below mentioned 5.4 commit.

Reported-by: Nucca Chen <[email protected]>
Cc: Cong Wang <[email protected]>
Cc: David Ahern <[email protected]>
Cc: David S. Miller <[email protected]>
Cc: Jakub Kicinski <[email protected]>
Cc: Jamal Hadi Salim <[email protected]>
Cc: Jiri Pirko <[email protected]>
Cc: Jiri Pirko <[email protected]>
Fixes: 62794fc ("net_sched: add max len check for TCA_KIND")
Change-Id: I66dc281f165a2858fc29a44869a270a2d698a82b
Signed-off-by: David S. Miller <[email protected]>

lib/nlattr.c

@@ -828,7 +828,7 @@ int nla_strcmp(const struct nlattr *nla, const char *str)
 	int attrlen = nla_len(nla);
 	int d;
 
-	if (attrlen > 0 && buf[attrlen - 1] == '\0')
+	while (attrlen > 0 && buf[attrlen - 1] == '\0')
 		attrlen--;
 
 	d = attrlen - len;