rx: work harder to notice and handle MorePackets request

rx: work harder to notice and handle MorePackets request

in particular, we did badly at handling kernel requests for
more packets, but other cases did not properly keep packet
stats either. attempt to globally better handle demand for
more packets.

Change-Id: I88837fed880f582444221ec53d280ca4070b607d
Reviewed-on: http://gerrit.openafs.org/1978
Tested-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Hartmut Reuter <reuter@rzg.mpg.de>
Reviewed-by: Derrick Brashear <shadow@dementia.org>