rxrpc: Merge together DF/non-DF branches of data Tx function

Merge together the DF and non-DF branches of the transmission function and
always set the flag to the right thing before transmitting.  If we see
-EMSGSIZE from udp_sendmsg(), turn off DF and retry.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Eric Dumazet <edumazet@google.com>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: linux-afs@lists.infradead.org
cc: netdev@vger.kernel.org