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 <nuccachen@google.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Jakub Kicinski <jakub.kicinski@netronome.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Jiri Pirko <jiri@mellanox.com>
Cc: Jiri Pirko <jiri@resnulli.us>
Fixes: 62794fc4 ("net_sched: add max len check for TCA_KIND")
Change-Id: I66dc281f165a2858fc29a44869a270a2d698a82b
Signed-off-by: David S. Miller <davem@davemloft.net>

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;