rxrpc: Fix key reference count leak from call->key

In the Linux kernel, the following vulnerability has been resolved:

rxrpc: Fix key reference count leak from call->key

When creating a client call in rxrpc_alloc_client_call(), the code obtains

a reference to the key. This is never cleaned up and gets leaked when the

call is destroyed.

Fix this by freeing call->key in rxrpc_destroy_call().

Before the patch, it shows the key reference counter elevated:

$ cat /proc/keys | grep afs@54321

1bffe9cd I--Q--i 8053480 4169w 3b010000 1000 1000 rxrpc afs@54321: ka

$

After the patch, the invalidated key is removed when the code exits:

$ cat /proc/keys | grep afs@54321

$