afs: Don't truncate iter during data fetch

Don't truncate the iterator to correspond to the actual data size when
fetching the data from the server - rather, pass the length we want to read
to rxrpc.

This will allow the clear-after-read code in future to simply clear the
remaining iterator capacity rather than having to reinitialise the
iterator.

Signed-off-by: David Howells <dhowells@redhat.com>
Tested-By: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
cc: linux-cachefs@redhat.com
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/158861249201.340223.13035445866976590375.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/159465825061.1377938.14403904452300909320.stgit@warthog.procyon.org.uk/
Link: https://lore.kernel.org/r/160588531418.3465195.10712005940763063144.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/161118148567.1232039.13380313332292947956.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/161161044610.2537118.17908520793806837792.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/161340407907.1303470.6501394859511712746.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/161539551721.286939.14655713136572200716.stgit@warthog.procyon.org.uk/ # v4
Link: https://lore.kernel.org/r/161653807790.2770958.14034599989374173734.stgit@warthog.procyon.org.uk/ # v5
Link: https://lore.kernel.org/r/161789090823.6155.15673999934535049102.stgit@warthog.procyon.org.uk/ # v6