rx: Use _CLASS_RECV_CBUF in rxi_ReadPacket

rx: Use _CLASS_RECV_CBUF in rxi_ReadPacket

Currently, the userspace rxi_ReadPacket() allocates additional space
for the incoming packet (if needed) with the RX_PACKET_CLASS_SEND_CBUF
class. But we are allocating buffers for receiving packets, and so we
should be using the RX_PACKET_CLASS_RECV_CBUF class.

This is clearly a mistake from OpenAFS 1.0, since all other
packet-receiving code uses RX_PACKET_CLASS_RECV_CBUF, and all other
users of RX_PACKET_CLASS_SEND_CBUF are for sending packets.

This mistake doesn't actually matter for most cases, since the packet
class given to rxi_AllocDataBuf() is ignored for non-KERNEL, and this
code is userspace only. The only time this is actually used is for
UKERNEL. For UKERNEL, this mistake could possibly cause us to
prematurely restrict how much space we allocate here (since the
_SEND_CBUF packet quota is larger than the _RECV_CBUF quota), and
trigger a sendCbufPktAllocFailures event.

To fix this, use the proper RX_PACKET_CLASS_RECV_CBUF class instead,
to match other packet-receiving code paths.

Change-Id: I94077d595102560375d12c766d176b87d427b735
Reviewed-on: https://gerrit.openafs.org/15336
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>