If rx->conn is not NULL, rxrpc_connect_exclusive() does not
acquire the transport's client lock, but it still releases it.

The patch adds locking of the spinlock to this path.

Found by Linux Driver Verification project (linuxtesting.org).
Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: David Howells <dhowells@redhat.com>