rxrpc: Don't lock call->tx_lock to access call->tx_buffer

call->tx_buffer is now only accessed within the I/O thread (->tx_sendmsg is
the way sendmsg passes packets to the I/O thread) so there's no need to
lock around it.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org