rxrpc: Extract the peer address from an incoming packet earlier

Extract the peer address from an incoming packet earlier, at the beginning
of rxrpc_input_packet() and thence pass a pointer to it to various
functions that use it as part of the lookup rather than doing it on several
separate paths.

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