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
$

Fixes: f3441d4125fc ("rxrpc: Copy client call parameters into rxrpc_call earlier")
Signed-off-by: Anderson Nascimento <anderson@allelesecurity.com>
Co-developed-by: David Howells <dhowells@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeffrey Altman <jaltman@auristor.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Eric Dumazet <edumazet@google.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
cc: netdev@vger.kernel.org
cc: stable@kernel.org