rxrpc: Reduce the use of RCU in packet input

Shrink the region of rxrpc_input_packet() that is covered by the RCU read
lock so that it only covers the connection and call lookup.  This means
that the bits now outside of that can call sleepable functions such as
kmalloc and sendmsg.

Also take a ref on the conn or call we're going to use before we drop the
RCU read lock.

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