net/9p/client.c: fix potential refcnt problem of trans module

When specifying trans_mod multiple times in a mount, it will cause an
inaccurate refcount of the trans module.  Also, in the error case of
option parsing, we should put the trans module if we have already got
it.

Link: http://lkml.kernel.org/r/1522154942-57339-1-git-send-email-cgxu519@gmx.com
Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: David Miller <davem@davemloft.net>
Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Ron Minnich <rminnich@sandia.gov>
Cc: Latchesar Ionkov <lucho@ionkov.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

net/9p/client.c

@@ -191,6 +191,8 @@ static int parse_opts(char *opts, struct p9_client *clnt)
 					 "problem allocating copy of trans arg\n");
 				goto free_and_return;
 			}
+
+			v9fs_put_trans(clnt->trans_mod);
 			clnt->trans_mod = v9fs_get_trans_by_name(s);
 			if (clnt->trans_mod == NULL) {
 				pr_info("Could not find request transport: %s\n",
@@ -226,6 +228,7 @@ static int parse_opts(char *opts, struct p9_client *clnt)
 	}
 
 free_and_return:
+	v9fs_put_trans(clnt->trans_mod);
 	kfree(tmp_options);
 	return ret;
 }