rxrpc: Increase the size of a call's Rx window

Increase the size of a call's Rx window from 32 to 63 - ie. one less than
the size of the ring buffer.  This makes large data transfers perform
better when the Tx window on the other side is around 64 (as is the case
with Auristor's YFS fileserver).

If the server window size is ~32 or smaller, this should make no
difference.

Signed-off-by: David Howells <dhowells@redhat.com>