afs: Check for fatal error when in waiting for ack state

When it's in the waiting-for-ACK state, the AFS filesystem needs to check
the result of rxrpc_kernel_recv_data() any time it is notified to see if it
is indicating a fatal error.  If this is the case, it needs to mark the
call completed otherwise the call just sits there and never goes away.

Signed-off-by: David Howells <dhowells@redhat.com>