af_rxrpc: Remove incorrect checksum calculation from rxrpc_recvmsg()

The UDP checksum was already verified in rxrpc_data_ready() - which calls
skb_checksum_complete() - as the RxRPC packet header contains no checksum of
its own.  Subsequent calls to skb_copy_and_csum_datagram_iovec() are thus
redundant and are, in any case, being passed only a subset of the UDP payload -
so the checksum will always fail if that path is taken.

So there is no need to check skb->ip_summed in rxrpc_recvmsg(), and no need for
the csum_copy_error: exit path.

Signed-off-by: Tim Smith <tim@electronghost.co.uk>
Signed-off-by: David Howells <dhowells@redhat.com>