rxrpc: Convert rxrpc_txbuf::flags into a mask and don't use atomics

Convert the transmission buffer flags into a mask and use | and & rather
than bitops functions (atomic ops are not required as only the I/O thread
can manipulate them once submitted for transmission).

The bottom byte can then correspond directly to the Rx protocol header
flags.

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